The first step is the easiest: cut two of the 2" x 8"s to 75" and two of them to 57". These directions are for a regular double bed. I made the frame to be bigger than the mattress by one inch all the way around. For a full rundown of common mattress sizes, see the Wikipedia page here: http://en.wikipedia.org/wiki/Mattress
So, the long sides are one inch longer than the mattress, and the short sides are four inches longer than the mattress, to accommodate both one inch of play on the mattress and one and a half inches on either side to overlap and create the joint with the long sides.
To reinforce the corners and create a spot for the legs to attach, I put a brace in each corner that was made from one of the leftover scraps of 2" x 8". Just cut the biggest right triangle you can out of the material. Make four of those.
Lay out the box on a level surface.