First of all, i have lost ONE game out of hundreds i have played (in normal -_- ... but i don't want to play ranked without a team of five...) and even when i lost, i was positive, got legendary, and had oodles of assists due to early laning phase.
You need to understand that she is an ASSASIN not anythng else. she can initiate, BUT you need to be able to escape if things go awry. before you initiate, make sure 'w' is on CD. that way you can un-debuff yourself and leave whenever. early gamem you need to try to stay away from them (except to land your 'e') and 'q'q'q'q'q'q' until your pinky hurts. let the people in lane do the tanking/huge dps, you're there to make sure they don't get away, via exhuast and intense 'q's.
post level 6 you need to let the enemy push, hurt whoever they're in lane with even, then ult in and destroy them quickly. Ev has imense initial damage and she's near impossible to escape (even under turrets)

You need to take all the jungle, go b, get basic boots THEN gank, unless you absolutely have to do something else. if you can, take wolves/wraiths again. gold is your friend. SO IS BLUE. anyways, you need to make sure your adc is fed, and if you can, feed mid and make sure top is ok. but tell your top, if they're not god, to be careful and dont take risks. mid too if you're not confident in them. that way you can get your adc a couple kills and then focus on mid/top. if your adc is ok, so is the game. ALSO counter jungle whenever you can, AND ward their jungle and make sure their jungle cant function, and their mid never gets blue. or their adc getting red.

by now you should be able to kill anyone who gets caught OFFGUARD AND ALONE. wards help this SO much. so invest in them. make sure if they start to use vision wards, you IMMEDIATELY get an oracle's. you need to practice not dying. so discretion with both your 'w' and exhaustion. you NEED to learn it, also with your ult, it pays to wait for more than one person to ult on, unless its a high priority target getting way under a turret. anyways, you need to start getting kills when its appropriate, and believe me, WARDS HELP.

Basicly you need to now be a sneaky *** more than ever. by now the enemy team will be wise to you, and hate you, and kill you everytime they can, so use that. divert the focus from your adc/bruiser, and you need to basicly run in, ult, hurt some people, MAYBE kill the squishy, then LEAVE with your 'w' and wait till you're needed again, trust me, you will be, and you're worth WAY more alive than dead. you need to sneak around them and make sure nothing goes wrong, from BEHIND them. that way there's no escape for them. if it goes bad for your team, 'w' in and 'q'. a lot. try to save whomever you can with slows and maybe kills. you also STILL need to be warding. it makes taking drag, red, blue, and baron away from them the rest of the game and keep it in your control.

Hunter's Machete- Makes jungleing impossibly easy. In fact, it makes it possible (in conjunction with hp pots) to take the whole jungle with a mediocre leash.
Sheen/Lich Bane- Evelyn's 'e' makes her att spd increase, soooo the lich bane is a great way to harness the buff it gives, while still benifeting fully from ap items.
Rylai's Scepter- Makes assassinations stupid easy. no one will ever escape, or catch you. it also provides health, WHICH YOU NEED.
Death Cap- Gives huge dmg output, wich makes Lich bane demolish everyone.
Huanting guisse- AP dmg means nothing if you're not getting through with dmg, plus it now builds into a ridiculous ap item which burns them for 5% of their hp (great for tanks and additional dmg)
Abysall scepter- again, ap dmg means NOTHING unless it gets through, and protection against those pesky ap's is always nice.