Get the development version of Gajim. Install git (either a console tool or one of the gui wrappers, like Git for Windows) and download Gajim sources. You can read more about how to use git with Gajim.
Create an XMPP account specifically for testing. It will be easier to test things if you will use a small XMPP account without all your friends. Also, you won't annoy them ;-)
Create a different profile with that account. Use gajim -p test or ./launch -p test and set up all things.
Start Gajim with debug logging python3 -W default ./launch.py -p test -l gajim=DEBUG