Find a biboumi instance e.g. irc.cheogram.com

On xmpp join #channel%irc.server@biboumi.instance (e.g. #gnu%irc.libera.chat@irc.cheogram.com)

For authentication:

/MSG NickServ IDENTIFY <your-password>

More biboumi instances, set automatic authentication (you need ad-hoc commands which gajim supports and dino not yet) etc, here and here