doing a small patch on a unsupported piece of software, but that was nearly trivial.
That means you've already started. ;-)
I think it's hard to give recommendations regarding learning of RE in general, as I mainly do it a "scratch an itch" and "learn as you go" type of thing --- whether you need to interface with an unknown file format, change the behaviour of a piece of code, change a message in a UI, automate an aspect of a web app, etc. the exact knowledge you'll need will vary widely. However, in all those circumstances the basic idea is to gather knowledge about the target to get a vague understanding of how it works, and then dive in with the specific tools/knowledge you've gathered.
I want to learn. I've played around a bit with r2, doing a small patch on a unsupported piece of software, but that was nearly trivial.
Can you recommend any good places to get started?