I tape up the edges and then paste over, sand and paste again. Usually enough...make sure that you vacuum your wall before you paint (Yes, you heard me right...vacuum your wall)...using a moist cloth doesn't cut it to remove drywall dust, and unless it's off...your paint will peel. Good luck!
BTW...#1 advice...measure twice, cut once
#2 - leave about 1/2 inch on the bottom...don't let it touch the floor. You'll need the room for expansion and floor/carpet etc...
#3 - do it sloooooooowwwwwwwlllllyyyyy, and carefully.