How to import custom public rooms?

I'm new to retros and I've been looking everywhere and I can't seem to find any help on how to import custom room models. Specifically, I was aiming at importing a welcome lobby. I'm running r63b and I've read in some places that you have to use certain commands, but these commands never seem to work. They all include a room model code and image code, and I have no idea where to put these codes in my hotel. Can anybody point me in the right direction? Thank you.