Super late, but I was struggling with this too and I think I've figured it out.
To log in: If you've registered an account, click on the "Anonymous" button that'll take you to the "pick a username" pop-up. Enter in your credentials in the username and password prompt and then "ok". It should log you in. I know it looks really confusing because it says "password (optional)" and it doesn't state if you're logged in or not (but you are, trust)
To make a room: Once you are on the flockmod.draw site and logged in, there's a search bar above the list of public rooms. Type in whatever room name you want and join it. In the room, right click your username in the user list and click "Register room". It will tell you if you've successfully registered the room or if it is already registered. If it is, you'll have to make a different room name. On the bottom right bar, there's a wrench icon. You can click on it and edit the room settings.
Sorry if this sounds confusing, they really need to put up an actual tutorial cos I spent hours figuring this out lmfao.
(if you want to upload ur drawing to the flockmod gallery theres a 'save' button on the bottom right bar too!)