first u need to know what u want it to look like that is hardest thing to figure out, u need to know the width of your rear axel outside of tires, the width of cab, the width of your fram, how big u want your sub fram, how long you want it to be, how wide, how much finder you want to see, do u want the sides broke or not and if u do how much angle how long, do u want the the mach to have a step down and how muck, r u going to put a goose neck ball in the middle of it, how big do want to your boxes and how many

hope that helps some
zach