Start with the kernel srpm. Do a: rpmbuild -bp kernel-*.src.rpm Replacing * with the approriate version and release. Then go into the directory where the kernel was extracted and generate your patch.
The trick here is to remember to extract the BUILD directory once with the above command, rename that directory (I add "-original" to the end), then extract a 2nd copy, and make the changes to that. This makes it easy to extract the diff.
With the latest Fedora kernel SRPM's the first run of the prep step will make a copy of the mint files to a "vanilla" directory, and if you do a subsequent prep, it just copies the files from the vanilla directory instead of extracting and patching from the SOURCES directory. So you can use the vanilla directory as the baseline to create your patch from.