If you haven’t yet commited the changes, then:
git diff > mypatch.patch
But sometimes it happens that part of the stuff you’re doing are new files that are untracked and won’t be in your git diff output. So, one way to do a patch is to stage everything for a new commit (git add each file, or just git add .) but don’t do the commit, and then:
git diff --cached > mypatch.patch
Add the ‘binary’ option if you want to add binary files to the patch (e.g. mp3 files):
git diff --cached --binary > mypatch.patch
You can later apply the patch:
git apply mypatch.patch