Step 1: Draw your footprint.
I used a waxed paper box as the basis for my line width. I had the logo on a magazine cover as reference, and used a ruler to help me judge the width and height of each letter in reference to each other letter. It's not a perfect rendition, but it's pretty close.
Step 2: Make the walls of the mould.
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.