Hello, I'm xKarmel and this is my first guide. I'll show you my Twitch's build and try to explain some things. So let's go. BTW sorry for my English :)

Pros:
+ Very fast attack
+ Huge amount of damage
+ Stealth (short duration but enough to surprise them :D)
+ Easy to kill escaping enemies because of Venom Cask and Expunge

Cons:
- Need a good teammates
- Twitch is not a tank
- Focused in team fights
- Not so good in escaping if you don't have Flash

Twitch is AD carry so:

Marks: Greater Mark of Desolation

Armor penetration is that what he needs. I don't prefer Greater Mark of Attack Damage, because I can get lot's of attack damage from items bought.

Seals: Greater Seal of Armor

These runes are good for early levels. Why not Greater Seal of Defense? Because if you will have more defense early, you won't be killed so fast, but you can kill, and if you success you can dominate late game.

Glyphs: Greater Glyph of Magic Resist

Why? Same reason as seals.

Easy to explain. If your champion is AD carry, you need to put your points in offense. I put 21 in offense and 9 in defense, because some HP will be useful in early game. Don't forget that attack is the best defense.

1. Flash - Twitch needs that! Remember, it's his only way to escape, but if you won't you will have more time to use Ambush. This spell is also good to chase running enemies.

2. There you choose. I can only suggest. If you prefer chasing enemies choose Exhaust. If you don't choose Ignite. If you want you can get Ghost or Teleport. They are useful too.

I prefer maxing as first Expunge. If you want to use it try first to hit as many times with your auto. It's a really good skill.
Secondly, max Ambush. It's good middle and late game for escaping, hiding and grants good amount of attack speed. Try to watch if your enemies didn't bought Oracle's Elixir. If they did, try to avoid roaming.
As last max Venom Cask. This skill has good slow effect and great range. You should use it to secure many kills.