Below is pretty much how I did it:
Step 1:
Buy an Ender 3. It's cheap and there are tons of upgrades out there. Start printing stuff from Printables/Thingiverse in order to understand how slicing and printing works. Learn the jargon by researching issues you run into (don't just ask on forums, search for yourself).
Step 2:
Find some upgrades for the printer through Youtube, Reddit and the likes. Buy (or print) them and upgrade your printer. Run into issues, fix them and start understanding how the entire thing works.
Step 3:
Print this [0] model and watch this [1] video on how it was designed. It shows really well what constraints you're working with when designing for 3D printing and how to work around them. Then, start designing your own prints in whatever CAD software you want to learn. I started with Fusion 360, but anything is fine, probably.