If you have made your own improvements to the ispCP code base, or if you just fixed some bugs and you want to contribute them to the community. A patch would be the right way to do so.
If just one file is affected by your changes, create the patch like this:
<cli>
diff -uN original.php new.php > filename.patch
</cli>
If more then one file is affected, try this command and compare the whole trunk directory.
<cli>
diff -urN original_directory new_directory > filename.patch
</cli>
Don't forget to update your trunk to the newest revision before creating the patch.