[Stolen from forum]
Hey peoples, this is Jo5h doing a comprehensive guide on Poppy, the Iron Ambassador. Poppy is arguably one of the best champions late game but has a unique play style. This guide covers AD Carry Poppy, AP Nuke Poppy, and Tank Poppy. It covers playstyle [Roaming, Laning, Jungling], team fights, and other various things.

Pros:
Natural Tankyness due to passive
Insane burst capability
Very good ULT if used correctly
Often underestimated
Has a mini-ghost
Cons:
Very bad farmer early game
Mana-starved early game
No ranged attacks
Easily harassed
Suspectible to CC

As mentioned earlier above, Poppy can fill the roles of AD Carry, AP Nuke, and Tank.
Here are the separate pros/cons.

AD Poppy

Pros
Naturally Tanky
Insane constant DPS
Very Fast Movement

Cons
Expensive Build

AP Poppy

Pros
Insane Nuke Damage
Able to 2 Hit carries
Can do the job without being interfered due to her ult

Cons
Banshee's can negate your ULT.
After your combo, you're very vulnerable
After your combo, you don't do damage

Tank Poppy

Pros
Very hard to kill due to her passive
Still does good damage
Decent initiation

Cons
No AOE moves
No Hard CC

So question is, which Poppy are you going to play? You can choose after buying your sheen.
Does your team lack a hard carry and you just want to rape face endgame? Pick AD
Does the other team have one very fed carry? Pick AP
Do you have no tank? Pick Tank.

Standard Pen for AD or AP depends. Your devastating blow is considered magic damage but as AD poppy, you want your normal attacks to hit about as hard as Q. Poppy is a little mana hungry which is why mana regen is there. Last of all CD per level to spam Q more.

AD Poppy
Standard 21/0/9 Set.

AP Poppy
Standard 9/0/21 Set.
Grabbing the mpen.

Tank Poppy
Again, standard 0/21/9 Set.

and
Ignite works with your Ultimate and is Poppy's only ranged move.
At level 18, ignite does 410 * 1.4 = 571. Useful for finishing off people with escapes.
Ghost is a must for Poppy. I prefer ghost over flash because it has a lower CD, and it lasts longer. With movement speed, paragon, and ghost you should be outrunning any or all champions. It can be used offensively and defensively. If you find yourself ganked you can live by using your ULT on someobody with no hard CC's and running away.

Other viable spells.

Flash can be used to close distance, position a charge, and escape over walls. I prefer ghost due to less CD and longer duration.

Use it to reveal units hiding in brushes, then punish them with your charge.
Also a great spell overall for map awareness.

Because Poppy's only form of CC is ramming into a wall, the slow is nice but I find it useless late game. Poppy either kills the opponent too fast or can get the slow from Red and Trinity.

Heal? Yes heal. Poppy is often in the front lines and when your about to die, pop heal! Heal + Passive makes you hard to kill. I find myself taking hits even with low hp due to the fact that the damage will do half damage on me.

I don't really recommend this because Poppy already runs really fast but you can use it to get behind the enemy which sets up a nice charge.

Valiant Fighter - Any damage dealt to Poppy that exceeds 10% of her current health is reduced by 50%. This does not reduce damage from structures.
This is what makes Poppy Tank viable. It is also very useful for saving you when you're near death. I've been saved many times from a Karthus ult or a Cait ult from this.
If you still don't understand what it does, here's an example.
At level 1, you lane against Annie. Her fireball does approx 90 damage. You have around 500 health. 10% of 500 is 50 so her fireball's damage is reduced to 45.
Devastating Blow "Q"
Poppy crushes her opponent, dealing attack damage (+0.6) plus 20/40/60/80/100 and 8% of her target's maximum health as magic damage. The bonus damage dealt cannot exceed 75/150/225/300/375.

This is Poppy's bread and butter skill. It does great damage throughout the whole game. It scales off AP and procs with sheen. Spam it as much as you can when your attacking a champion. Best time to use this is after an attack animation.
Paragon of Demacia "W"
Passive: Upon being hit or attacking an enemy, Poppy's armor and damage are increased by 1.5/2/2.5/3/3.5 for 5 seconds. This effect can stack 10 times.

Active: Poppy gains max stacks of Paragon of Demacia and her movement speed is increased by 17/19/21/23/25% for 5 seconds.

Increased armor + damage AND movement speed? Very useful for Poppy. Use it before initiating every time. The movement speed buff is like a spammable mini ghost.
Heroic Charge "E"
Poppy charges at an enemy and carries them for a short distance. The initial impact deals 50/75/100/125/150 (+0.4) magic damage. If they collide with terrain, her target takes 75/125/175/225/275 (+0.4) magic damage and will be stunned for 1.5/1.5/1.5/1.5/1.5 seconds.

This skill has a variety of uses. It can close the distance between you and an enemy, it can disrupt channels, it can push an opponent towards your team, and best of all it can stun. The damage more than doubles if you collide them with terrain. This includes destroyed towers. Very deadly in the jungle and in side lanes. You can also use this to go through walls. This skill also scales off AP.
Diplomatic Immunity "R"
For 6/7/8 seconds, Poppy is immune to any damage and abilities from enemies other than her target.
In addition, Poppy's attacks deal 20/30/40% increased damage to the marked

This is what makes Poppy so amazing. She can be caught off guard and run away alive. You can use this on their support/useless member on the enemy team and your untouchable for 8 seconds. You can also use this for "Anti Carry". Poppy does increased damage to whoever she uses this skill on. With her already insane nuke, Poppy can and will two hit kill squishies all day.

Her skill rotation in a fight is W > R > E > Q and then use any skill you can to proc sheen/trinity/lich. The proc can be used every 3 seconds so time your skills to maximize DPS.

AD Poppy

Core Items

Trinity Force
Must for AD Poppy. It has the sheen proc, movement speed, and a slow. Get this item ASAP.
Berserker Greaves
For AD Poppy, the increased attack speed helps. If you feel as you don't need the aspd, go for mobility.
Phantom Dancer
Movement speed, Attack speed, and crit. Three important things to AD Poppy. I sometimes grab two or three of these items if I can afford it late game. Movement speed is very good on Poppy. It enables her to chase, run, and position her charges.
Bloodthirster
At full stacks, this item gives 100 AD and 25% Lifesteal. By this time, you should be doing tons of damage. This lets you survive in team fights a lot longer as you smack around every champion you see. Very useful in staying alive.
Insert Tanky Item Here
At this stage of the game, you can pretty much beat any champion 1v1, do insane damage in fights, and keep your HP up. The enemy team will begin to target you after your shield goes down. This is when I build a tanky item. If their carry is AP, get FoN. The HP Regen is very good on Poppy combined with her passive. If their carry is AD, I recommend Randuins. It has a slow, CDR, and decent Def.
Infinity Edge
If the game is not yet over, you can get an Infinity Edge to do more OP damage to everyone. You should be critting consistently and doing as much as your Q.
Other last items could include Wriggle's if you want to solo Baron or Tiamat to be able to kill minions insanely fast.

AP Poppy

Core Items

Lich Bane
Makes your Q hit very very hard. I don't get this item till I get some AP because Sheen does the job decently early-mid game.
Sorc Shoes
Mpen, need I say more?
Deathfire Grasp
This item works with your ult and is a good item overall for AP poppy. It has CDR and Mana regen. I grab a Lucky Pick as Poppy builds are often expensive. Using your ULT, DFG, E and Q should instantly kill most champions.
Zhonya's Hourglass
A main problem with AP Poppy is that she's very vulnerable after her one combo kill. Using Zhonya's lets you refresh your cooldowns and keeps you alive to combo another victim.
Rabadon's Deathcap
I think this item is just overkill on Poppy. I recommend getting a Void Staff is the other team is starting to stack some MR.

**Spell Vamp
I'd rather go more damage than spell vamp but it does help you live a bit longer.

This item is too expensive for my tastes but the active works with your ult and the slow is always helpful. It also provides spell vamp to keep your hp up after your combo.

Good item if you have a few AP people in your team. Provides spell vamp for all.

Tank Poppy

Core Items

Sheen
This item enables you to still do insane damage while being super tanky. Rush this first.
Boots of Swiftness
Poppy needs movement speed to initiate and run around saving lives.
Insert all tank items here
Are they heavy AP?
FoN, Banshees, Abyssal Scepter etc etc.
Are they heavy AD?
Thornmail, Randuins, Sunfire etc etc.
Are they mixed?
I recommend FoN, Randuins, GA.
FoN gives mspd and health regen
Randuins gives an AOE slow, and cdr
GA makes you very hard to kill.... again.

Side Notes

As a starting item, I usually pick Sapphire Crystal because she has a low mana pool. Get either this or Level 1 boots. Boots enables you to run faster in case you want to roam or position charges. It is vital that you get Sheen as soon as you can.

Optional items

Poppy is very susceptible to CC. But I find that other boots are a lot better. Use your ult to make your immune to most CC's in teamfights.

I've seen Poppy's get this because her build is very pricey. I don't get this because as AD Poppy, you can jungle after getting sheen/vamp scepter and farm free lanes. Your very hard to gank because of your ult. As AP poppy, you have kage's lucky pick. Only get this item if you can't really farm/get kills.

Laning with Poppy is actually very easy. Never ever take a solo lane unless you absolutely have to. Poppy is very horrible at farming unless your playing mid-late game AD. To lane with Poppy, you need to go with someone that can either solo lane, heal you, or have some form of CC. Try to reach the brush earlier than your opponents at level 1. Camp the very tip of the brush and wait to see if your opponents walk in. Right before the opponent walks in, walk out and slam him into the wall taking out a quarter of his HP. If you have a good partner, first blood is very easy. Now as the laning face progresses, your going to want to stay in the bushes as much as you can. If anybody approaches the bush, you can use your Q to hit them and stun them with your E. Do not try to farm because almost every time, you will get harassed. Another tactic is wait for an opportunity to slam the opponent into the wall. Near the area where the bush ends, you can slam the opponent into a wall as long as they aren't dead in the middle of the lane. Play aggressive in a smart way. When a person is around half HP, you can walk around to the river, pop Ghost and use your E + Q combo to kill them. Oftentimes, many people will hug the brushes for safety. This is to your advantage. Wait for the moment they walk out or you can see them and slam them into a wall.

TL:DR
Don't try to farm, focus more on hogging the bushes and using Q/E to punish them for going near you,

I admit, I'm fairly new to roaming as Poppy and tips would be appreciated.
As roaming Poppy, tell your team you are going to roam so you can get matched up with someone that can solo lane. I take boots first and my heroic charge. If they have a jungler, I camp in a nearby bush and can often get first blood as there are many walls to slam against. If they have their whole team, pop ghost and run the hell away. With your passive, you should be able to live unless you get stunned for a long time. I usually lane until 2-3 getting my Q and W. If I can't get an easy kill in the lane, I check the map for anyone near half hp. Once you hit six, you can tower dive enemy champs if they escape. Use your ult, charge them at the tower, and if your low, let your teammate get the kill so you can walk away safely. If you have the hp to spare, go ahead and take the kill. Note that the shield disappears if your target dies.

To Gank Mid Lane

Wait for them to overextend past the middle and enter from one of the side entrances. (Not the brush entrance, the one closer to their tower) Use your W to get the movement speed boost and charge them either towards one of the "very small mountain" thing by the river or towards your teammate. With your E and Q, you should take out about half of their HP. If your teammate has a stun, let them stun first as it enables you to have better positioning.

To Gank Side Lane

This is actually very easy. Most of the time, the opponents will either be near the brush, or overextending. Come in from the river with the mspd from W and if their using the brush to escape, wait for them to leave the brush and slam them into the wall. As always, if your teammate has a stun/slow let them initiate.

Poppy is also a decent jungler, after getting sheen you can take out all the camps without wasting too much time. If you find yourself having to fight, the jungle is the best place to be due to the abundant amounts of walls. AD Poppy can take out dragon solo after trinity force and solo baron at full build.

With any Poppy build, you always want to initiate from the back or the side. This is because the carries will always be behind the tanks and your job is to take out the carry. With your kit, you can initiate from the back, ult on their carry and watch the rest of the team waste ults on you. Try to shove their carry or an overextended towards your team. As AD Poppy, you'll want to use your ult on somebody with soft/no CC. If they do a good amount of damage, get them real low but do not kill them. When they start to run away, immediately target the next closest squishy. Your slow from trinity and your high movespeed should keep them from running away. As AP Poppy you want to come out of nowhere and take out their carry before the opposing team can react. If you fail to kill one person, you have failed and now worthless in the fight. Remember your combo is R, DFG, W, E, Q, Repeat, remembering to proc lich/sheen as much as you can.Tank Poppy You will be very hard to kill and can initiate. Use your ult on their carry to do increased damage. Use your ignite and hopefully the carry will start backing up. Save your heroic charges for disrupting channels like Kat, Nunu, Warwick, Malz. Spam your W and Q as much as you can.

Know when to use your ult. Your ult has a longer range than any of your skills, so I only recommend using your ult when you can charge/ catchup due to movement speed.
Good use
1v1 Fights
Ult + Ignite near dead opponents
Champions that are in range for a slam
Champions with no escape
Tanking Towers
Killstealing Baron
Escaping a gank

Bad use
Anybody with Banshee's. It will waste your ult.
Anybody with spell shield.
Anybody that can escape.
Anybody that can render you useless e.g. Malz, WW

If your escaping from a gank, do not ult on anyone that has slow/stun. You will be stuck there and your ult only lasts 6-8 seconds.

Always proc your sheen/lich/trinity as much as you can and abuse your movement speed. Always save your ult as a last resort or a for sure kill and remember not to farm unless it's safe. Every time I see a Poppy try to farm, I harass with ranged abilities/auto attacks. Jungle when you can because both buffs are excellent on her. Abuse your damage! Take a few hits to charge opponents into the wall and smack their face with Q.

That about sums up my guide for Poppy. Hopefully I covered everything about her. Any questions or unclear statements leave me a comment. Any tips is much appreciated.'

[Creds to Lictor for testing stuff out]