Welcome to the Guild, Jalyha. Given your goal of mapping one or more of your worlds, a starting place might be to pick a tool ( software you like or have or can afford, which can produce the kinds of maps you prefer). Then pick a tutorial that will build some skills - not necessarily even a style you want, since any exercise with your toolset will improve its general usefulness. And you'll get the most mileage out of the tutorials if you tackle them in Work-in-progress threads- interactive development will answer questions and give new ideas -- and we're a pretty gently helpful bunch.

Digital tools aren't your only choice by the way - plenty of us prefer physical drawing / painting.

You have a huge advantage in that you've already richly imagined your worlds. That does leave challenges , since total random terrain generation isn't a full option, but random*ness* still can do a lot of the work - you don't have to design every river bend and ridge line.