Well like any adroid phone each is a little different, and the Modaco forum is the palce to go.

But in general the step are
1) "Root" the phone. This can be as easy as downloading a rooting app from the market, or more involved. It is an unlocking step to allow you to do the following:
2) Install a revised bootloader. This ~5MB program allows you to change the main system image. Kinda like MS Dos was for PCs, if you remember that.
3) Install Android. This ~90MB file is the main program. It will be specific to your handset.