As it has been said, start with doing one small map. I've also wanted to do an episode first, but it was just too ambicious for me, and i've abandoned it (only one map survived
). It's good way to get some experience, gain some creativity and then do more and more ambicious projects.
And also patience is one very important thing for mappers. Lots of times you will have to sit over one map, fixing some stupid bugs, adding some lightning, changing textures when you'll find out that they suck, changing whole sectors when you see like they're too cramped or something. You have to give much time to the map, so it will be closer to perfection. I'm saying it according to my own experience, and i can say this way of thinking can change your maps to be better.
And also i try not to force the mapping - if i have no ideas, i get a break until some ideas come to my head
If i can suggest you something. Take one map of your episode, which you think is the best, and spend some time over it, until you find it's really great stuff. Then post it on this forums, people will tell what's to change, what you have to avoid, and generally give you some advices.