A patch is a file which contains the differences between original files and updated files in such a way that other people who only have the original files (e.g. NetworkDLS) can evaluate your changes and update their files with your changes.
Creating a Patch:
Personally, I use TortoiseSVN because it allows me to manage source code right inside of Windows Explorer. Once you've gotten the latest source-code for a piece of software, made some changes (and done at least some testing), creating a patch with TortoiseSVN is pretty straightforward.
1) You'll want to right-click in the folder/directory where your modified source code resides, go to the TortoiseSVN sub-menu and click "Create Patch".
2) Then you'll be confronted with a list of files which you've changed. Select the files which you want to include in your patch and check OK.
3) Save the patch file in a place where you can easily find it.
4) After saving your patch file, TortoiseSVN will give you a break-down of what the patch contains. Give this file the proverbial once-over to ensure it contains only the changes which you want to include in the patch.
5) Possibly the most inportant step, upload the patch file to NetworkDLS. We can't wait to see what you've done!