Go to the corners and drive framing nails just an inch or so into the framing. Stretch strings across the wall at the top, center and the bottom. Tie them to the nails. Adjust the strings out from the wall framing on the nails until they're just barely touching the most studs. These are your high tops. The studs that do not touch by at least 1/8 -1/4 inch need to be ferred out to just touch the string like the high tops. You can use ripped 2x material or plywood strips, the most economical way is to use 2x6s and cut several fron one board. It's easier to build them out than to cut them.
Welcome to the forum!
[URL="http://www.houserepairtalk.com/announcement.php?f=39&a=6"][size=3]The Ten Commandments of House Repair Talk[/size][/URL]
[URL="http://home.bellsouth.net/p/s/community.dll?ep=16&groupid=289528&ck="]Square Eye's home page[/URL]