I first made Illustrator drawings of the letters. I wanted to make 100mm high models based on the upper case letters of the Futura typeface. The Illustrator files would have to include the fold lines and any tabs needed for gluing the models together.
I found it was a good idea to differentiate the fold lines and the cut lines by placing them on different layers in the Illustrator file.
I also found it helpful to colour code these lines differently (Green for folds, red for cut lines).
From Illustrator, I exported the image to my CraftRobo cutting machine. I specified that the layers I had chosen for 'folds' should receive a perforation cut and all of the rest of the lines be treated as cut lines.
I loaded up the CraftRobo carrier sheet with a sheet of 160gsm white card and set the machine going. There's something very satisfying about sending your data to a robot to do all of the tricky work for you...
I could get two letters on each sheet of A4 card and the CraftRobo took around two minutes to cut each sheet.
Folding the letters together is pretty easy (especially with the pre-scored perforated edges for the folds) and it just takes a dab of Pritt stick or similar to glue the tabs together.
UPDATE 17/01/2017 WITH NEW VIDEO ADDED!
I've now made a 'how to' video showing how to make these letters. This video shows how to use Adobe Illustrator to create the letter templates and then a demo of how to cut the letters (with scissors), score the folds then fold and glue the letters.