The best way to start developing on the code base is to use GNU Arch's capabilities of handling private branches. The idea is that you can develop on a branch that you completely own, and easily synchronize with the official repository. Please note that this synchronisation works in both directions: you can keep up to date with the latest changes, but you can also make your own changes available so that they can be merged in the official repository.
Setting up a private branch
There is a short tutorial on Arch's wiki: http://www.gnuarch.org/gnuarchwiki/mini_LocallyBranching