Early game you need to determine the enemies damage and range to see how easily you can poke with W. Most mid lane champs can't outtrade your full combo. Make sure to abuse the **** out of your fleet and corrupting potion to out sustain keeping people in lane long enough for you to get a good back. Speaking of which here are some solid numbers to back on.

650 - boots and dark seal
1300 - lost chapter
700 - boots and a tear crystal
750 - (into ranged) boots and doran's shield.

Also don't be afraid you try an sneak drag with a melee jgl that is at least lv 4 as you two can clear them ridiculously fast.

You want to be looking for 2v2 and 2v3 skirmishes, with a melee ally, as you excel in those. Also don't be afraid to engage on lonely squishies, as you can 100->0 most of them, if they don't have zhonya's. Also, aggressively invade the enemy jungle when you can even with out the jgl item you clear extremely quickly. More or less you are playing as a second jgler. Also when you have the knowlage feel free to invade and take enemy buffs.

I get this pretty often so here are a few common points I end up making

I was on a ESDL team and was moved to mid after some roster changes and needed to learn mid. However, I didn't play any mid laners at the time and had just been on a Sej kick so I tried it out.
After that I found I wasn't interested in being a jgl main anymore and started looking at the other roles, so I decided I'd try mid and top.
She has nearly 0 winning matchups top and you can impact the map better from mid.
It really ****s with enemy jglers who are trying to counter pick her.

Hi, I have over 100 games on sej mid about 70% of which is in ranked. My ign is Sejuwuani I'm usually hovering around b1-s3 as I get burned out after that. If you ever want to get ahold of me for questions or a vod review/coaching on Sej mid specifically (I don't feel confident like I can offer any expertise in anything else you can't find from a better source) then you can find me on discord at THE BIG GAY TOMATO#1400. Also #blm #transrights.