How to submit a patch to ModemManager?

[Elias Rudberg]

Hello ModemManager developers,

I would like to submit a patch to fix this issue:

https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/issues/257

Are there some rules or recommendations about how to submit a patch to 
ModemManager, like rules for coding style and/or what commit messages 
should look like, if everything should be squashed to a single commit, 
things like that?

Best regards,
Elias