Hi! :)
Welcome to my first guide. ^^
Talon is my main (one-trick) from pretty much when I started playing, so that would be more than 4 years ago, but I took long breaks in between.

I was rated as the 7th best Talon in EUNE and have over 1M mastery points on Talon, so I believe I'm quite experienced with him and after his rework I decided I'd make a guide.

Talon's Passive

Talon's spells Wound enemy champions and epic monsters, stacking up to 3 times. When Talon attacks a champion with 3 stacks of Wound they bleed for heavy damage over time.

Talon's Q

Talon stabs a target unit. If they are within melee range, this attack does critical damage. If they are outside melee range, Talon will leap at his target before stabbing them. Talon refunds some health and cooldown if this ability kills the target.

Talon's W

Talon sends out a volley of daggers that then return back to him, dealing physical damage every time it passes through an enemy. The returning blades deal bonus damage and slow units hit.

Talon's E

Talon vaults over any terrain or structure, up to a max distance. This ability has a low cooldown, but puts the used terrain on a long cooldown.

Talon's R

Talon disperses a ring of blades and becomes Invisible while gaining additional Movement Speed. When Talon emerges from Invisibility, the blades converge on his location. Each time the blades move, Shadow Assault deals physical damage to enemies hit by at least one blade.

I chose Rake as my first ability for early AoE poke damage + CS. It's also a very good slow for denying ganks or catching enemies.
Noxian Diplomacy second because it's one of your main sources of damage and it also allows you to proc Blade's End at level 2 along with Rake.
Assassin's Path third for escape potential in case of early ganks or tower dives. You can choose to get this fourth and Rake third for more poke and kill potential if you think you can score a kill at level 3.

Flash - Probably the best Summoner Spell in the game. You can use it in your combos and for escapes, invaluable to Talon.

Ignite - Securing kills earlier in the game or just more burst late game.

Other spells are a bit lackluster on Talon.

Pros:

-Very high burst damage
-High mobility and map presence
-Invisibility from Shadow Assault
-A lot of escape potential
-Good split push
-Unpredictable engage/flank patterns

Cons:

-High mana costs
-Squishy
-Vulnerable to CC and peel
-Scales off late game

Marks - Lethality + AD. I have seen people running both full AD and full Lethality on these , but I believe a mix of them gives the best results. AD is very good for pushing towers , CS-ing and scales better early. Lethality helps you transition from early to mid game smoother , since Lethality outscales AD as the game progresses.

Seals - Armor , if facing an AP Mid you should get Scaling Health instead.

Glyphs - Magic Resist.

Quintessences - AD , it helps with CS-ing and your level 2 all in. I have seen people get Lethality Quints but Ad Quints outweigh their damage early-mid game and you won't really need that little bit of lethality later.

Youmuu's Ghostblade should be rushed first if you plan on roaming a lot and barely staying in lane and Duskblade of Draktharr if you want more lane power, buying Boots of Mobility to help with roaming in both cases but especially helpful if you went for Duskblade of Draktharr first to compensate for the lack of out of combat movement speed.
Maw of Malmortius should be rushed first if you are against an AP laner that poses a threat.

( Tiamat Build)
Rush Tiamat first for wave clear because it boosts your roaming potential by quite a lot and then later sell it for your last item. You should look to complete items in this order : Duskblade of Draktharr > Youmuu's Ghostblade > Black Cleaver > Lord Dominik's Regards > Edge of Night / Guardian Angel.

The build order listed above works in most cases, but feel free to change it depending on the situation. With this build you should run Sorcery secondary with Nimbus Cloak and Absolute Focus.

The normal build with inspiration secondary lets you get into the game sooner because you don't have to buy Tiamat, so you reach your power spike earlier.

Buying Black Cleaver after your first or first two Lethality items helps a lot because Talon is really squishy and if you are ahead it's recommended building this so you don't lose your lead to a very good enemy team focusing.

You should finish your boots somewhere after or while building your Youmuu's Ghostblade / Duskblade of Draktharr. It's good upgrading them if you're getting really fed and snowballing so you can transition your lead by ganking other lanes. If the enemy team has strong CC or maybe an AD/AP heavy composition you should look to get the other options I listed in the Boots section.

As Talon you have plenty of ways to engage an enemy champion.
Here are the ones you should use:

1)(Fast Combo) Rake > Noxian Diplomacy > Shadow Assault immediately after casting Noxian Diplomacy, while you are mid air so you get the Auto Attack reset on landing, proccing Blade's End

2) Shadow Assault > Noxian Diplomacy > Rake > Auto Attack for Blade's End + Ignite if necessary

2') Shadow Assault > Flash > Noxian Diplomacy > Rake > Auto Attack for Blade's End + Ignite if necessary

3)(Surprise Attack) Assassin's Path over terrain > 1) , 2) or 2')

4)(Extended Combo) Rake > Noxian Diplomacy > Auto Attack for Blade's End > Shadow Assault > Noxian Diplomacy if the target can escape or didn't die yet.
5)(Used for early game kills when you don't have your ultimate or if the target is very squishy and has no sustain, you choose the order of Q and W depending on situation) Noxian Diplomacy / Rake > Rake / Noxian Diplomacy > Auto Attack for Blade's End + Ignite if necessary

Talon has a bit harder early game mana wise if you poke a lot, but if you can get kills early that's not going to be a problem later.

At level 2, if you followed the ability sequence , will be a big powerspike. Even though Talon has high mana costs, his early game is extremely good, for a simple reason: you can all in on people at level 2 because of your high base damage.

Firstly, you should start by using Rake for the slow and for 2 ticks on your passive. Then Noxian Diplomacy for third tick, Auto Attack to activate the bleed from Blade's End and Ignite, also proccing Electrocute.

Mid game is where Talon shines.

If you got fed or at least some kills through the early game, this is going to be a breeze. Although Talon, and Assassins in general, get rather useless when behind so you should make sure to get fed, because Talon is a very snowball-y champion and that's why getting kills and roaming is very important.

You'll find yourself dealing a lot of damage in teamfights, and even if forced to leave, your teammates should be able to finish them off.

If you manage to get fed or have a good number of kills until late game you're just going to keep getting more. A fed Talon is a scary Talon.

Sadly, if you get to this part of the game you'll start scaling off so you should try to end the game and help your team as much as possible.

In teamfights, you should focus on assassinating high priority targets, like the enemy Mid laner, ADC or Jungler. (Only if the Jungler is some kind of assassin and is not tanky)

Also, you should never initiate teamfights with Shadow Assault, they will back and when you come off invisibility they will CC you and kill you. (Worth mentioning to also never initiate teamfights with anything, not only ulti. You should be looking to flank after your team engaged, if you are the engager they need to follow up immediately or you'll probably die.)

Thanks for reading guys. :)

Be sure to leave some feedback and suggestions for my guide! ^^