First, great to hear you want to get involved in coaching rugby
How to learn fast? I am afraid there is no quick way .... Try to read, watch and talk rugby as much as you can.
Perhaps a different angle: make the learning process part of the team development. Ask the girls what they feel is missing. I have lots of experience but I do too! Ask players who watched the game, ask the ref after the game. It helps to get everybody focused. Turn it around and make this a strong point of you.
Rugby is about confrontation. You have the ball and ask the defender: "Are you going to stop me?". A challenge that is no different for boys than girls, men or women. I have two girls in my boys team! Nobody has a problem with that! Everybody is happy they enjoy the rugby and face the challenge.
Most of all create an atmosphere where your team can develop. My best advise would be to read the John Duda article on my website.