Basically, as anyone would most likely told you, they practiced, learned from mistakes and looked at reference maps and all of that. They then tried new things, etc. If you try to get better it is most likely gonna happen over time. However, if you feel like you can't do shit all and your never gonna do anything..then chances are your not.
All comes with practice.
I still can't believe what some of my old maps looked like.
>.<