The text file you've created should be saved to the installation folder of Disk Cleaner, in the subfolder 'plug-ins'. For example, save the file in C:\Program Files\Disk Cleaner\plug-ins. Give the file a useful name and make sure that the extension is '.dct'. Otherwise Disk Cleaner will simply skip your plug-in.
Well, this is all there is to it. Note that you can open all the pre-installed .dct files in Disk Cleaner's installation folder with notepad, to see more examples. Of course, you are welcome to comment on this or
to ask questions. Please use the forum for that.