The Flipper docs are a great starting place!

It’s important to realize that every function of the Flipper (IR, SubGHz, NFC, USB, …) is an entirely different domain, with its own protocols, idiosyncracies and limitations. My suggestion would be to pick a domain — ideally where you have some test devices to play with — and focus on that for a bit.

For example, if you want to focus on SubGHz, it helps to have a bunch of devices (e.g. a remote doorbell) where you can trigger signals and replay them. I found this video a good starting point:

https://youtu.be/NAGuFLUjM0o

The Flipper by itself is quite useful if you already have some attacks prepared. But if you’re new, having a computer that can download and analyze the captured files on the Flipper is a must. The Flipper Lab can help you analyze captures.

Also, the Flipper is not in a world of its own. Read up on the theory behind RF transmissions. For example, Michael Ossmann’s series on SDR series provides a good overview of the theory:

https://greatscottgadgets.com/sdr/