Welcome to my Miss Fortune guide! This is my first guide so leave comments/suggestions below and I'll see what I can do.
Miss Fortune is one of the many ranged Attack Damage (AD) champions in League of Legends. She excels in dealing Area of Effect damage with her ultimate. Many teamfights depend on this type of damage! Tristana, may do more single-target damage, but Miss Fortune will do that damage to their entire team. With a simple calculation we can show that Area of Effect damage is pretty important!

We assume that your ultimate, Bullet Time, hits 3 enemies.

• Each bullet of her level 2 ultimate deals 85+0.5*100=135 magic damage, while hitting about thrice a second: 3 * 135 = 405 damage.
• This is applied to 3 enemies: 3* 405= 1215 damage per second. That is a HUGE amount of damage while under level 16.

Hasta la vista, Tristana!

Pros:

• Great last hitting.
• Good farming ( Make it Rain)
• Champion with one of the largest base range.
• Slow
• Above 2 makes for: Great kiting ability
• AoE ultimate
• Damage skill, for free bonus damage. And active for attack speed. ( Impure Shots)
• Good passive for great mobility around the map. ( Strut)
• Okay harass, with Strut + Make it Rain.
• Has a crazy laugh

Cons:

• Squishy
• No stuns
• Not the best 1v1 (AD) champion!
• Strut is not usefull in battles
• Double Up hits pretty randomly

• Ghost
  I pick ghost so I can kite better, chase faster and also escape some nasty situations.
• Flash
  You can never have enough survivabilty Summoner Spells for a squishy carry. You will encounter plenty situations where flash will save your life, while ignite or exhaust wouldn't.

Other summoner spells that may come in handy in certain situations are:

• Cleanse Against a team of stunners, blinders and slowers.
• Exhaust For use as anti-carry spell, this is very effective. Also serves as a damage increaser and escape ability, while slowing your chaser.
• Teleport Blue pill to home and to the lane in a flash? Teleport delivers you this capability. Lanes being pushed: Teleport. Friends getting ganked: Teleport. You get the idea, it gives you a large map presence when handled properly. Also check Elementz' Teleport comments.

Strut Double Up Impure Shots Make It Rain Bullet Time

Strut is your passive ability. It helps you get around the map way faster than other champions. You can also use it to kite champions that cannot damage you from afar. But be aware that minion hits will also cancel your passive! Next comes Double Up which I mainly use to hit 2 enemy champions when they stand close to eachother. Using it on minions in the laning phase, while hoping to hit the champion is mostly a waste of mana, because -unleveled- Double Up doesn't deal much more damage than an auto-attack. Pro tip: Use Double Up between auto-attacks for bonus damage! Following is Impure Shots, your auto-attack skill. It has a nice passive that stacks, making you deal magic damage on top of your normal auto-attacks. You can see it as the old version of Malady. It's active gives you a very decent amount of attack speed and reduces healing. You have your own Ignite, use it! Mundo uses ult? No problem, one Impure shot will make him pay! Make it Rain, this is the ability I use most in any match. Early game it's so good for a number of things. 1:You can farm minions from afar; 2: You can push lanes with it; 3:You can escape ganks with it; 4:You can/must use it for harass. Try to hit both the caster minions and the enemy champion at the same time. And if they are close and running, hit them with an auto-attack while they're slowed. Your ultimate Bullet Time is your signature move. It is what makes Miss Fortune really different from other champions. Very important to notice is that you mustn't use it when a teamfight hasn't started yet, or has just begun. Wait till both teams have committed to the battle and then use your ultimate for MAXIMUM LAUGHTER (I really love crazy laughs).

For those interested, I quote one of the comments below that, I feel, describes how to use double up well.

There are two main routes you can take with Miss Fortune.

Route Make it Rain

This route is the easiest of the two, thus it is also the one I will advise to new players of Miss Fortune.BUT it is also the safest! If you are expecting to get harassed in lane or if the enemies team has a strong roamer/jungler, you may opt to take this route. It allows you to farm minions fast and from a much larger range than auto-attacks. Run in, do Make it Rain on minions and back off (inside XP range). But make sure you don't push the lane too much, because the further you push the more danger you are in.
Also Make it Rain is very usefull for averting ganks, if you level it, the slow becomes a lot better and that's why it allows you to escape better than Route Double Up.

Route Double Up

This skill sequence is more suitable for advanced players of miss fortune. It allows you to harrass your lane opponent excellently. If your opponent is hiding behind minions, Double Up the minion in front of him and it will bounce toward your enemy dealing120% of your attack damage, plus another 35/70/105/140/175. This is quite potent if used correctly. That is because it denies your opponent the ability to stand behind his minions safely. So he either has to go to either side of the minions (where he still isn't safe you are at the right angle), or stand in front of the minions, which is very aggresive and susceptible to harassing with auto attacks and ganks.

I always start off with a Doran's Blade. It just gives so much value for you money. You may want to consider a Doran's Shield if you'd like to play really defensive, but that item is less usefull than a Blade later on. Also buy a Health Potion, so you can farm more before going back to Home. Tip: If you have lost 300 health or more, use your health pot. You want to keep your health high for possible ganks and you won't regen fast anyway early game. Next off are Boots and more health pots. Boots are required really soon, because as a ranged carry you will have to keep your distance with melee champions, because they will outdamage you when you're not fed. You may proceed to step 3 if you have 435g or more left. To finish off the Early Game and make you able to gank, is the B. F. Sword. Later on we will build this into Infinity Edge so last hitting will become a breeze, your ult will be supercharged and harass is your thing.

Early game you will want to keep last hitting as good as possible. Harass with auto-attacks is cool, but keep in mind that minions will attack you as soon as you auto attack a champion. Even if you get 2 hits off, the minions will possibly deal that damage right back at you. So a better strategy is to use Make it Rain as it will NOT agress minions (spells don't draw minion aggro). You may want to start ganking about now, if you feel like it. But Mid Game is where you will really shine in ganking.

Welcome to Mid Game, gank time as I would interpret it. After getting to about level 8-10, let someone take over mid and recall for boots. Mercury's Treads are my favorite, because casters are usually my main threat, but Boots of Swiftness will also do if you're in a good position to kite. If you have some money left, grab Pickaxe or Cloak of Agility. You have tried to gank lanes, now it's time to push! If your ganks have been succesful, push with the champions in your current lane. Dual lanes are good for this, because even if one person comes to the turret, you're still with 3 people. Map Awareness is the keyword here, so consider buying a Sight Ward if your thinking of pushing. In any case, last hitting is still a top priority so you can get your Infinity Edge for some real punching power. This item alone is worth recalling for as it gives you so many good stats! Also grab a Vampiric Scepter if you have to money to spare. Next up is Zeke's Harbinger. Get a Recurve Bow first, preferably, but a Vampiric Scepter will also do. As long as you're building Starks. Now to finish it off, we'll finish building our Zeke's Harbinger and move towards more team oriented play. Consider grabbing another B. F. Sword for your Black Cleaver before finishing off Stark's, if your team doesn't have any other champions that benefit from the aura. See below for my calculations on getting Starks' Fervor, as requested in the comments.

Mid Game is your time to shine! Try to draw the attention of your jungler, so he will come gank mid or take over the lane while you get ganking. Move in behind the enemy you're trying to gank, while assuring that you won't die. Check their summoner spells! If you've pinged the map, so your teammates know you're attacking, then you may initiate the fight. Your Make It Rain is very important, try to position it as such that the enemy will have to walk trough it to get to safety, but make sure it hits your enemy, by putting at least the edge over him. If you can gank their solo lane, that is preferred, because it will cost them XP and you can more easily kill 1 person with 2 ultimates. But plan your actions wisely, summoner. Fortune doesn't favor fools!.

Reason for gettin Stark's after Infinity Edge

If the enemy team has 2 enemies that are getting any amount of armor, Black Cleaver will rip right through that. With Stark's and your rune you will already passively shred 50 armor, but with Black Cleaver you will shred, at MAXIMUM LAUGHTER, 100 armor! If the enemy has many bulky champions you may choose to get Madred's Bloodrazor before Black Cleaver. It's attack speed will stack with that of Impure Shots and Zeke's Harbinger, for MAXIMUM LAUGHTER. And when you finish Black Cleaver after Madred's Bloodrazor, your laughter will be pretty deadly... Are you getting focused and having trouble keeping your distance from the enemy? Don't fear, The Frozen Mallet is here! It gives you a huge lump of health, but more importantly a permanent slow for maximum kiting/laughter. Their magic champions are fed? They have 3+ magic champions? You are getting jumped by kassadin all the time? Then get Force of Nature. It is now cheaper than Banshee's Veil. And gives you movement speed, health regen and maximum magic resistance. Even more than a Banshees. Is Master Yi sticking his sword where it doesn't belong? Then get Guardian Angel for more armor and survivability. However, make sure that you never stand in the middle of their team. Then even your Angel won't save you.

It is endgame now. You are Miss Fortune and not a solo pusher, so try to stick with your team. Stay behind your tank, never walk into the enemy team and try not to initiate. If they come storming at you, back off, start kiting. You dont want to end up int he middle of their tanks while their dps are firing on you. Of course focussing tanks is not always a good idea, but if you cant reach their carries without putting yourself in the middle of the brawl, you should shoot the tanks. As a DPS you are in fact suitable to destroy these Titans.

I cannot stress this enough, but if you're experienced enough you can decide for yourself what the best items are. This is after all, only a guideline! There are other items that would fit into this build:

A Youmuu's Ghostblade could fit into your build if you want that little bit of armor penetration early game by building The Brutalizer. You also get some money by Avarice Blade Cooldown reduction, for increased farm/harass with Make it Rain, and huge Attack Speed buff for a few seconds. A Phantom Dancer gives a few stats you want. You want critical, as it stack well with your Infinity Edge. You want attack speed for more auto-attacks. And last but not least you want movement speed, which is quite an underrated statistic. Not to mention it is pretty cheap at only 2895 gold. I would advice you to build this after you've finished Infinity Edge, or maybe grab Zeal a little early. Next up is Manamune. Great early game item. It isn't cheap though at 2110 gold, it's more expensive than a B. F. Sword. However at a base mana of 896 plus the bonus from manamune, topped at 1000 = 1896 mana. 2% of 1896 mana = ~39 attack damage. This is when Tear of the Goddess stacks are maxed, which takes quite some time. In the end you'll get 39+20=59 attack damage and an almost infinity supply of mana. Tip: Only buy this item if you are having mana problems in many games as we are not getting any other mana items. Ah, Bloodthirster, the second most powerfull attack damage item in the game, behind a Sword of the Occult, both with max stacks. It also gives a very decent amount of 25% Lifesteal at MAXIMUM LAUGHTER. However, each time you die, you will have to farm those stacks back up which may be kind of a hassle sometimes. And keep in mind that an Infinity Edge gives more auto-attack damage, because of its Critical Chance.

I made this build to be used in conjunction with Route Double Up. It capitalizes early game on the damage from Double Up, which also uses the Vampiric Scepter to gain some life off your double ups, because it applies lifesteal.
I've matched the early game items, with an anti-tank build end game. With last whisperer penetrating 40% of your targets armor you will still do damage against highly armored targets.
However, you can also use the end game items of Route Make it Rain if it stuits the situation better, because the enemy has a lot of squishy target who are not building armor items. You have a brain, so use it!

I hope you liked reading my guide, please remember to post comments below. And to you whiners, trollers and *****ers out there: Watch your mouth kid, or you'll find yourself respawning at home