Cloning the repo and making changes
On Windows the repository should be cloned in the directory %APPDATA%\Blender Foundation\Blender\2.90\scripts\addons\ (replace 2.90 with your Blender version here) under a subdirectory that must have the name blender-to-smf. This subdirectory name makes sure that Blender is able to find the add-on when it starts.
Once the add-on is set up like this you can simply switch between dev and master branch if you'd like. The dev branch could be less stable than the master branch.
Note Cloning the repository adds all files. If you want to keep a clean directory you should install from a release zip.
After making changes you'll have to reload the add-on in Blender. This means closing Blender entirely and opening it again.
Important From 0257abb the SMF exporter supports reloading in Blender without the need to restart. See
Reload Scriptsandbpy.ops.script.reload.