Take your plans to Kinkos, an architect or engineer and get two copies made. Cut one around the outside shape of your guitar.
Take a piece of 3/4 MDF or press board and cut into four piece a little larger than one half of your guitar. Clamp them together (I like to use 1/4 inch flat head bolts and counter sink the heads), true up one side (which will be the center line) and trace your plan onto the MDF. Cut out with a band saw (a saber saw will work) and you will have something that looks like this
Flip them over and put some pieces of MDF across the top and bottom to splice them together - something like this
I also put a couple of little spacers on the sides at the waist. You want the mold a little less tall than the height of the sides (again, get that from your plans, the neck end will be thinner than the tail). I keep the center pieces that get cut out and use them in my side bender, but you can also cut them into small pieces that will make the waist expander. Chisel a couple of notches and use a turn buckle like so
Many other ways, some very sophisticated but this works for me.