Basically what you have to run is diff with the options: urN.
Let's suppose that Super Grub Disk without the changes is found at: original_version folder. Let's suppose that Super Grub Disk with your own improvements is found at: improved_version
diff -urN original_version/ improved_version/ > /tmp/sgd_myfeature_patch.diff
The filename has to end in .diff extension and with the syntax: sgd_myfeature_patch.diff where myfeature is the name of your feature or what your patch does.
Please check that you do not send "enters", "intros" (new lines) inside patches (except when needed, of course).
You can also read: How to Report Bugs Effectively