How to create map?

Assuming that you have on a CD-Rom a digital map of London but actually want to adapt it for use as a GPS map in a GPS, do you know how this is done or what software you would require to do so?

It's done with very expensive software and a lot of time. Teleatlas create most base maps and sell them to companies like TomTom, who then provide what we have on our GPS devices.
Not something that is readily available for the home market - yet.