I don't have a any pictures of the early stages of making a house as I haven't had to make one in a while, but the beginning stages are similar to the last hill I made. I started with a large flat piece of cardboard then think about how I want the outer walls to look, then think about the inside of the walls. Then from that decide where I want the windows. I won't want the windows to be able to see from one side of the house straight through the other so I plan where to put the windows based on that. Then I just tape everything down to the base and glue it down. Make sure the height of the walls are bigger than your figures and that the windows are the right height if you want your figures to standing looking out the window or crouching and looking out the window.
Adding a second level adds more dimension to the game. I just folded a piece of cardboard to make the stairs, but I didn't use the right cardboard for it. What would've been better is that semi-thick cardboard that bends better at the folds, that would've made for flatter more even looking stairs. I suppose I could always update them later. The walls aren't terribly straight either, but as you can tell from the windows I set them up so that they couldn't see straight into the whole house.
I didn't used to paint the houses, but I have to admit the color does add a lot more to the landscape. They are pain in the butt to paint though. I did find that I liked the grey floors, but you can paint them whatever color you wish.