I believe knot-tying would be a great, life-long skill to have, especially being able to know which knots work best for different situations, like when the rope is thick and not very flexible or bendable.
I have lots of nylon rope and fishing line of various diameters, but don’t know where to begin and/or the best way to get started. My goal is to make this a long-term hobby and spend 1-2 hours per week improving my knot-tying knowledge, skill, and speed.
All of the above! You’ll need to practice and like others have said - an application. So get joining a group and absorb other kinds of media about it.