This process is new and we have little experience - we expect it will evolve over time, so please send any suggestions for improving the process.

Currently the idea is:

Check out the game from SVN. Make some changes. Make sure you've updated to the latest version of the code (and merged in your own changes). Create a patch, with svn patch (or the equivalent "create patch" feature in TortoiseSVN). Attach it to a Trac ticket (an existing ticket if there's a relevant one, else a new one), then post a new topic on the forum to request review. One of the core developers should then review it and maybe suggest some changes. When it's considered okay, they should commit it to SVN.

The person submitting the patch must agree that it is their own work (or else make it clear where it came from), and agree to licensing the work as BSD or GPL (we're not yet quite sure which we want...)