Building a Mold
While it is true that you can make a guitar with very few tools there are some tools and fixtures that really go a long way toward making the job easier and the result better.  A body mold is one of those things that is hard to do without.  Happily it isn't expensive or difficult to make your own.  Here's how I built the mold I used for this guitar. 
       
Make a tracing of the guitar from the plans included with the kit. Because I was making a copy of a guitar, I made a tracing directly from the guitar. Draw a line down the exact center of the body outline. Cut off the excess poster board, cut in half and compare. If the two halves aren't identical use the best one. 
If you plan to make more than one guitar make a permanent template. Use  plywood, masonite, aluminum, or plexiglass. Use the cardboard template as a guide. Here is the finished template.  I like Plexiglass  because you can see through it  when you're doing your layout and it is easy to engrave.
There are a gazillion designs for molds and they all work. Do a little research and choose one that makes sense to you. To make one like mine, cut 6 pieces of 3/4" Plywood that measure 10" x 24".  Use your template to transfer the outline of the guitar onto all six  pieces.
Use a coping saw, sabre saw, or band saw to  cut to within 1/16" of the line. Take one piece and saw or sand right up to the line. Take your time and make this piece perfect, it is your template. I've set the plexiglass template on the plywood to show the fit.
Use a flush-cutting bit in your router, clamp your plywood template to one of the rough cut pieces and and use the router to make a perfect duplicate. Once you have all six pieces cut you can glue them together.
  To lighten the mold and make it more convenient to use you can trim the corners and the sides.

Here's a neat trick for keeping the halves of your mold in alignment . drill a pair of holes through the joint and then glue a dowel in one half.

The other thing you need to make a  body mold work is  spreaders.  Inexpensive turnbuckles are mated with wood cauls shaped to fit the inside of the mold. I use three pieces of 3/4" plywood about 2" x 4". On one piece I trace around the eye of a turnbuckle.
A little work with a band saw or a coping saw cuts out the notch. Now trace the contour of the mold onto another block. In this case, at the waist.
Stack two blocks together and visit the band saw again. Here are the pieces before they're glued together.
Glued and clamped. The finished spreaders installed in the mold.
       

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