If you want to start with something simple then have a look at this software http://freespace.virgin.net/patrick.fox/codriver/ , note that it is designed to run on Psion 5MX but I thought the concept was quite good.
It worked by taking a file or files containing a list of directions with distances from the start point and as these distances were reached the display would change. The data was taken from MS Autoroute or the Psion Streetplanner and Routeplanner software but I guess it would work with any text file containing the data in the correct format.
Obviously it can't recaculate on the fly so if you take a wrong turning it doesn't work until you can tell it where you are on the original route, but for something basic I thought it was quite good.
Ray