yea 15mm. take it off like a bmx tire.

chain tensioners help too. not necessary, but they can help.

if you dont have chain tensioner, just pull the wheel as tight as you can and tighten the cog-side nut. it should just sit even and you just tighten the other side. at least that's how i've done it and it ends up straight. if i'm doing it wrong please tell me.