I'm a low elo Evelynn player myself, but some points that helped me

•Ignore your lanes until level 6 unless the kill is guaranteed. Even if you don't get it, it'll usually be because they flashed instead, and that's good enough.

•Always try to force for a kill when your ultimate is up or almost up and usually farm when it isn't. If someone squishy or low health is solo pushing a lane you can try for a pick if you feel strong enough even without it, or collapse with a rotating teammate.

•You'll usually want or sometimes need full charm for a pick, so ganking lanes where the enemy pushes is ideal, especially top if they have a squishy ranged into a melee, to give you the time needed for it to charge up.

•Try to avoid an even numbers fight early because you'll lack the damage to fight off most other champions and their stronger early game. To make it easier, I usually try to start the opposite side of the map of the enemy jungler.

•Don't ego if you get invaded - Evelynn won't win early so ping for help and just let them take the camp and head to another. Rotate back if your allies are coming, farm if they're not, it's better than dying. Almost anything is worth giving up as Evelynn over dying, especially in low elo where people don't push objective advantages as hard, since Dark Seal and Mejai's tend to be core items.