Using the letters (or images) you've drawn as a footprint, make walls to support the material you're pouring in (in my case, rice crispy treat batter). I used the walls of the pizza box, but any long strip of cardboard will do. You will want to make sure the strips are even in height, and that the corrugation of the cardboard is vertical for horizontal strips. This makes it easy to bend your pieces to fit around curves--each bit of corrugation will bend in the right direction. If your corrugation goes the long way, then your bends will be hard to keep straight.
Tape the long pieces of cardboard together if each individual piece is not long enough for the footprint shape, and tape down to the footprint on the outside for stability.