pretty long stick, huh?
I tried to keep it short cause long guides suck.

no sheen? no damage!
i pick sheen first (after and ), for nice harass or propably some early kills on damaged champs or squishies (dont overextend!).
sheen (or later ) are a huge source of your damage.
afterwards stack ad.

The monkey in this build is pretty squishy, so be careful and don't die. not dying pays off, even if you don't get any kills! But you are able to deal some nice damage in the beginning/mid-game.
When you get ****ed early game, you are propably ****ed for the rest of the game ;). But that is what happens to any champion.
Preferrably your laning mate has some kind of stun, slow or other cc.

Itemorder is messed up (usability lacking on the skill editor and own lazyness ;) ). but you should get the first three items (BUY SHEEN FIRST!). Trinity should be bought after at least one ad-item (but buy it before running out of space). lifeleech at some point is always helpful. then ad, ad, ad, ad, ad, ad.

Buying a Potion in the beginning may upset your laning mate as you will have to wait 10g to afford one and will arrive a little after the first minions on lane. In random games its sometimes like feeding the trolls if you do so and some reactions will honestly blow you away ;D . I did not yet figure out if it's better to tell your mates or not ;)

€: i was trying around with vampiric scepter first for fast wriggle's lantern which gives you red buff pretty early on alone for its passive. lifeleech and additional armor make you last longer in the laning phase. Some say Wriggle's is not too good for the monkey - i like it though (buffs and farming in lane easily give you a great opportunity to get certain items fast). after that go for Sheen immediatly. But.. and there is always a but: lifeleech will not work too well early on. You won't be able to leech up your health very fast. So lifellech may be more effective in the later game - but try it yourself and decide.
vs a heavy ad-tam you can consider to get thornmail (i usually dont get one, but sometimes its worth it).
sometimes its great to add frozen mallet but only if your team lacks cc.
usually you just wanna go for ad items, as this build is not tanky and more focussed on damage and assasination. In-and-out with max damage.

pre-ulti: e - autoattack - q - if possible another autoattack - w - RUN FOR YOUR LIFE!
if ulti is ready, use it after 2nd autoattack and then w out.
usually the skillorder is situational and if your lifebar lets you afford it, go for another e - autoattack - q - autoattack combo. there might be some delay inbetween ulti and 2nd combo, so take care not to die.

in def-tree i like the dodge, to avoid damage sometimes (i like it better than 1% health regen).
off-tree is my standard-build. but make your own decisions on the off tree.

flash and ghost for escaping.
exhaust is good too.. you know why.

other summoner skills i dont find to useful. but as always its personal preference.

Im not sure about the runes, but i dont think armor or resistances do to well as you will usually not support these stats with items.
Comments appreciated on that.