Step 2: Design the wheel on the cardboard
My design (with instructions for use) is available as a (free) PDF at http://www.cathyschords.com/keytransposer.htm
Just print the first page straight onto the card if your printer can take the card; or you could print it onto paper and the photocopy it onto the card (I find this is easier as the copier takes the card well).
If you're doing your own design, remember that you'll need 12 equal segments (on a larger and small wheel) to allow for the semitones- see my picture. I used MS Word - a pie graph, then Wordart for the letters so I could rotate them as I wanted.