I have seen this done a few ways and was wondering what you guys though was best.
THe way I normally go about it is to measure the side's and add 1/4" for reveal. Measure accross the top and add 1/2" for reveal. I find the center of the top and tack that piece in place. Hang the sides and nail it all in.
I have seen people actually assemble it on the floor then lift it into place already together.