If you do not have developer access to QGIS cvs, or would like to submit modifications you have made to the QGIS source code using a "diff", you can generate the diff file like so:
cvs diff -u > some_diff.txt
Which will encapsulate any local changes you have made to the CVS checkout.
If you receive such a diff file, you can reapply it to your local sources by using the follwing command:
patch < some_diff.txt
provided that your current directory is the same as the directory that the diff file was created from. To avoid confusion it is best to always create diffs from the top directory. If the directoy path in the diff is different to yours, you can use the following to tell patch to perform the patch in the current dir. The -b option tells patch to backup changed files too..
patch -b -p0 < some_diff.txt