I chose these runes to Maximize and Balance twitch with AS and HP. Because extra AS is always nice, stack with Expunge, Your Passive and Debiliating Poison.

Twitch is always squishy. So with these HP Seals and Quintessences you get like 100 hp more. Expecialy at the start it's great... That's why I chose Greater Seal of Foritude, which gives you 5,35 bonus health, and Greater Quintessence of Fortitude, which gives 32,4 bonus health.

Twitch is an awsome ganker, so why not power up? That's why I chose to use Greater Mark of Alacrity, which gives you 1,7% bonus attack speed, and Greater Glyff of Alacrity, which gives you 0,76% bonus attack speed.

The end conclusion is:

9x Greater Mark of Alacrity
9x Greater Seal of Fortitude
9x Greater Glyph of Alacrity
3x Greater Quintessence of Fortitude

Offence;
Attack speed, Attack dammage, Armor penetration, Extra critical Dammage, Extra critical strike chance, Extra base dammage, Cooldown Reduction and Extra Magic Dammage.

Utility;
Increased mana regeneration, Increased experiance and Of course, Greed. (This is an expensive build ;) ).

As Twitch, you should rely on the dammage of Expunge. In fact, it's ment to be your "Killing Blow". So I say, max it first, so you get the full potential as early as possible.
Second, i'd say maximize Debiliating Poison. The slow will help you finish of your target(s) more easily.
Of course, maximize your ulti 3rd (Like you should with every champion, Need more explaination?).
There is 1 perticulair reason for maximizing Ambush last: It is Unneccesairy to stealth for like 60 seconds. Ambush at lvl 1 is Plenty. Only get Ambush for an ambush and mostly for it's bonus Attack Speed.

As for Summoner Spells, I would say Flash and Ignite. Flash for Escaping, Dodging, which you will be doing the whole game, and Ignite for when you just hadn't had that last shot.
If you're are fammilar and pro with Positioning, you could also swap Flash for Ghost. BUT, if you ARE NOT fammilar with good Positioning, don't. Flash will save your life.
If you still want Ghost, feel free to swap Ignite for it. But know that if you do, also change the masterys a bit, and beware of the 10 less Ability Power (Expunge will hit less this way).

Also good:

- Exhaust
- Ghost
- Teleport
- Cleanse

(Just what you like)

Never use:

- The Rest of the spells

Pros:

- Good Mid/Late game.
- Massive Multi Target Dammage.
- Easy farming.
- Awsome gank.
- Capable of Tower Diving.

Cons:
- Squishy.
- Pretty weak early game.
- Depending on Minion Farming.
- Depending on Early Game Results.
- Always short on Mana.

Twitch has to stack his passive (Look at Expunge) for the last hit. So I thought it would be neccesairy to atleast give him Massive attack speed.

So as item setup:
Beginning: I recomment Boots of Speed and 3 Health Potions, to get a movementspeedboost at the very beginning. But if you want first blood, you should probably buy a Dagger and a Health Potion, to get Malady first and be slightly stronger at the beginning. ALWAYS BUY MALADY AS YOUR FIRST COMPLETED ITEM! (Or else you wont make early-game kills, and be a noob with twitch).

2nd: Change your Boots of Speed in Boots of Mobility.

3rd: Buy a Black Cleaver, the passive, bonus attack speed and dammage will be perfect for Twitch. (It's like an item made for him i think...)

4th: Buy a Phantom Dancer, The attack speed, movementspeedbonus, and critical strike will help you getting more advantage of your Ultimate.

5th: Buy a second Phantom Dancer.

6th: Eather buy Frozen Mallet or Infinity Edge, Frozen Mallet is expecialy for the slow and the bonus health, and the Infinity Edge is for Hardcore dammage and critbonus.