Can I make my own maps?

Hi there.

I ride mostly off road these days and I was wondering if it's possible for me to chart the paths and tracks where I ride and turn them into a map that the Garmin 800 can use to navigate. Currently I plan circular routes to get back to the car but it would be nice to just ride then hit 'take me home' or something and it will direct me back to the car park via the forest tracks and paths. Currently if I do this I just get a straight line pointing to the car and have to guess the turns.

Thanks.