I just make cool looking areas and then connect them to other cool looking areas and slowly try and decide how they should fit together into a cool layout as I go. Sometimes I have some specific gameplay ideas I want to do, and I'll start there and design around that.

Planning everything first is a way better approach and gives much better results though... but I find it so boring to work on a level that I already know where it's going. 

I agree with Scampie. I have an additional problem where I find it hard to stay interested in a box map. It needs to look cool to maintain my interest and that's not always productive, I know, but it's how my brain works. 

These replies are very curious after reading the rock bottom frustration thread. 

I plan my map holistically, I map it out in a crappy paint program, labelling what I want where. The exact details get ironed out during the build, but I have most of the ideas in my head from the get-go.

Then as I continue I realise that my skills and my imagination don't correlate and I end up simplifying or giving up :)

As for the last question, the two are inseparable. I think "ooh gee, I'd like some zombies... I know a graveyard!" or "this is an excellent balcony, ahh, the perfect place for an ogre" 

Usually when I have a map idea in my head (sometimes a series of images, sometimes static) I just try to recreate it. If I am out of ideas, I imagine walking around the level and what areas it could connect to that would be cool. Most of the time I always think of something but some ideas just take a long time to actually create faithfully. 

I start by having the layout and items of a room or two in mind and then try to fit the lot together with a relatively good flow. Actual build starts with floors only after which I start filling in walls and architectural detail.