Firstly create a new Git branch:
git checkout -b BRANCH-WITH-CHANGES
Do your changes and commit:
git commit -am "My changes."
Now create the patch file by a diff from your new branch and the master branch to extract the changes:
git diff master BRANCH-WITH-CHANGES > EXTRACTED-CHANGES.patch
To test your patch, you can switch back to your master branch and apply the patch (the changes will not be commited, until you commit them in another Git command):
git apply EXTRACTED-CHANGES.patch