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.
To make it short and sweet, Evelynn is a ganker and a harasser early game with her stun. Make sure you don't lane alone and you move lanes constantly to gank when possible (Especially once you had Mejai's Soulstealer). Mid game you should have a better opportunity to get successful 1 on 1 ganks. A mix of ignite and stunning and ravaging correctly you can finish other champions fairly easy. By end game work with your team to stun while they open up, or vise versa.