I would recomment taking a peek at https://www.amazon.com/Conquer-girl-girl-fulfilling-submissive-ebook/dp/B005CIZ2FI/ref=sr_1_2?dchild=1&keywords=conquer+me&qid=1620983026&s=digital-text&sr=1-2 - I identified a lot with what the author wrote, and it is one of the few books that talks about brats.
A lot of things covered here but one thing I haven't seen is some insight into how to actually construct and maintain a dynamic. Kink is psychological, so maintaining D/s energy between each other is just as much about how you execute rules as what those rules are. For example, if your gf has a subby personality, she probably already does little things for you (we often call those acts of service in kink). She might bring you water when you're on the computer, tidy up, do the dishes without asking. Making those things into rules, or even just spontaneously ordering her to do them, turns those things from acts of love to acts of submission, and pushes you both into a D/s headspace.
Second, once you have her doing things in the context of your dynamic, it's important for you to see her. By which I mean, interact with the dynamic, remind her she's yours, make it clear you notice and value her submission. Submission needs to be seen. By which I mean, unless she's into a certain kind of dynamic, if you just give her a list of rules then call it a day, she'll do them out of a commitment to you, but if you don't enforce them or ignore her doing them she'll eventually feel alone. Remember the difference between submitting and keeping a rigorous lifestyle is that she's doing it for someone, so be a part of it too.
Speaking of rules it's worth remembering that once you go 24/7 and set rules, kink is no longer a thing she can turn off. It'll be ever-present on her mind. This isn't necessarily a bad thing, but it's worth remembering if you find yourself forgetting about the dynamic. She does not have that luxury, and you can hurt her badly in the process.
I've found Conquer Me to be one of the best resources on what makes subbie people tick. It's written towards your gf (and honestly she should read it too) but it's useful for anyone. My partner doesn't have as much of a subby side and she definitely struggled initially to understand and interact with it, so if that at all sounds like you I strongly recommend it.
This helped me a lot for thinking about submitting - https://smile.amazon.com/dp/B005CIZ2FI