If yours came marked with trim lines, use a Dremel with a cutting wheel and cut off the excess FG. If you have access to a belt or electric sander, use it to sand all of the rough edges. Lay the JP flat on the ground periodically to make sure you are sanding it evenly. Use some fine grit sandpaper to smooth out any imperfections to the outer shell. You may need to Bondo any low spots. You will need to get a wooden dowel rod to use for the axle (I can't remember the diameter). Cut out the holes on the sides and run it through until you have just enough sticking out of each end to attach the thrusters. Mark the dowel and cut off the excess. Once that is done, lay the sheet of styrene over the back and trace the outline onto it. When you go to trim it, leave a little excess on the sides since the concave of the JP will draw them in a bit. If you don't, you'll end up with gaps. Decide on how you are going to mount the straps to your harness before you glue the back on. I wrapped my straps around the dowel rod. Once everthing is finished, go ahead and primer it. I recommend painting all of the individual pieces BEFORE you glue/bolt them into place. I also recommend replacing the solid jets with plastic spray bottles (there is a thread on this topic).
This is kind of a rough explanation, and I am sure I am leaving steps out. If you need anything clarified, post the questions here.