Patches should be in unified diff format. If you have source code from Subversion, you can create a patch with:
svn diff > brlcad.patch
If you have sources from a source distribution tarball, you'll instead need to run:
diff -u -r brlcad.original brlcad.modified > brlcad.patch
Please review and/or edit the patch file to remove any unintended or unwanted changes. Try to make sure your patch can be easily applied to and compile with the latest BRL-CAD sources.
Thanks for the contribution!