Templates are just text files. The readme describes where the templates are located, so you know where to create templates, and you can use my templates as examples for the syntax. Or just install the ones in the repo and modify them.
As far as actually editing those distributed by SDF, I doubt it's worth it as opposed to just making your own as described above. You might be able to locate the SDF jar file which contains the template and edit them there, but then you'd lose all your changes whenever SDF updates.