A great description of contributing to Apache XML-RPC is available from the Jakarta project.
The preferred patch format is unidiff. The best way to generate a patch is by checking out the sources, applying your changes and running
Emails containing patches should be sent to the mailing list firstname.lastname@example.org, and prefix their subject lines with [PATCH]. Patches are also accepted via the issue tracker. The Jakarta project provides a description of life with ASF source code -- look for the Patches section.