Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
For those looking for a indepth guide, turn back.
The purpose of this guide is to simply outline a well rounded playstyle that works well in plat games without readers going TL;DR.
This guide is made assuming you know Evelynn mechanics (which is not much) well enough to focus more on strategy and tactics than straight up mechanics.
When to use this build.
Have at least one other friendly frontline.
Team needs magic damage.
Team can generally survive early game ganks without your help (mobile champions).
Enemy have medium or low level peels (don't bother if they have azir + thresh)
Enemy is not a hyper counter jungling jungler (lee sin, kindred)
Pros and cons of this build.
Early game: You're useless.
Mid game: Hextech gunblade and chilling smite gives you a near 100% kill rate on ganks, plus awesome burst to kill someone straight off with some help in 2v2 fights.
Late game: With runic echo and gunblade plus another AP item of choice, you can still kill an unprotected carry within seconds. Even if the enemy has great peelers, you function well as a frontline bruiser. With 45% CDR you deal considerable damage to bruisers/tanks.
Evelynn have no straight on fighting power early in exchange for the ability to bypass wards. This is called "balance". There are many ways to play around this weakness, and one of the way is to just straight up farm till level 5.
What to do
Make sure to deny vision in your own jungle by smiting the razor camp. Enemy not having vision of you forces them to play safe even if you just farm.
When you finally decide to gank, numeric advantage is absolutely necessary. DON'T GO FOR THE OBVIOUS OPPORTUNITY, or you will probably die to a counter gank, unless you are SURE the enemy jungler isn't nearby. You deal little damage and die to focused fire so fast it's not even funny. But, having your OP passive, it's quite easy to go unnoticed, and gang up on an unsuspecting lane bully.
This is the Brurst-Bruiser Evelynn power spike.
With gunblade and runic echos built, you can 1V1 most laners if they're at 70% health or has no ult. Your speed and slow from smite and gunblade and dark frenzy makes escapes nearly impossible even with flash, so any lane is prone for ganking.
What to do
Your job now is to strike fear into the enemy team, amass kills and forcing them to play passively.
Only go for ganks that has a high chance of yielding a kill. You must snowball hard, so ignore your "how to save this lane that is behind" impulses, instead think "how to destroy this lane that is already ahead".
Take free towers or free dragons(which means its uncontested/undefended), but otherwise ignore objectives that takes too much effort.
When teamfights start, you should be pretty tanky and ahead in items. If not, then you've already failed.
What to do
Observe the enemy team's playstyle. Do they invest lots of people to protect their carries, or do they go all in?
If they protect backlines: just play like a normal bruiser. Make sure to ult at least 3 enemies at the start of fight, the damage and slow deceptively powerful, while the shield also discourages focus fire.
If not: Go assassinate backlines. You have stealth, making this a lot easier than even the conventional assassins like talon or zed. Being tanky, you can safely rush in, take on a carry, and back out after your attempt. Even if you failed, you'll disrupt the enemy team pretty nicely.