Here is my guide to mid lane Evelynn. This is basically the kind of build you would use if you know what you're doing and is no way safe for new players of this champion, it's squishy and dangerous if you let yourself get caught.

With Evelynn's runes I go for magic damage with the Marks and Glyphs, early durability with the Seals and mobility with the Quints.

For her masteries I get all of the basic magic damage skills in the Offensive Tree and then movement speed for mobility from the Utility Tree.

With the items you start off with boots and health potions like any good player. You go straight for Kage's Lucky Pick for the AP and gold regen. After that you finish your boots for more mobility. At this point you should have more than 400 movement speed, which is very helpful for Evelynn since her play style involves a lot of chasing. Then comes Death Fire Grasp for more burst, Hextech Revolver for sustain, Sheen and then Lich Bane for even more burst, Rabadon's Deathcap for a massive damage increase, finish your Hextech Gunblade to increase your sustain and help chase those stragglers at the end of a fight, and then void staff for any magic resist the enemy might build.

The skill sequence is basically Hate Spike, Ravage, and then Dark Frenzy, while taking Agony's Embrace whenever available. When laning you should stick to hitting the enemy with hate spikes from afar while dodging skill shots or whatever else they throw at you. Don't be afraid to run straight at then and use Ravage and then chase with Hate Spikes, you have the extra movement speed for a reason. When ganking, you would open with Death Fire Grasp and immediately follow it with Agony's Embrace for a devastating combo and then finish them with Ravage and chase with Hate Spikes until you kill them. Dark Frenzy is great for escaping because it removes slows so don't always use it right away because if you fail to kill your target and refresh the cooldown you might not make it out.

Basically Ignite for extra damage and Ghost for chasing, you could take Flash as well but really combining Dark Frenzy with Ghost is much more useful than Flash, in my opinion.

Once again this build is very fragile and should not be used on people new to Evelynn and her play style. But if you know what you're doing this build can destroy entire teams. Please rate and comment, thanks for reading. :)