1. Create a hollow elevator shaft. (combining to the door if you want)
2. Create a brush about the same size as your door and place it where you want the elevator to be when it goes up.
3. Apply the 'func_plat' to the small brush.
4. With the just the small brush selected, press 'n'.
5. Type in 'height' as the key and for the value, it's rather confusing... so read carefully.
6. Here's the formula: (top position) - (brush thickness) - (bottom position) = height. So if I wanted my elevator to start at unit
(-1000) and go up to unit (0), then this is what the formula would look like: (0) - (8) - (-1000) = 992
7. So the value for the 'height' key in that example would be 992. If you want your elevator to start at the top and go down, you place the brush at the bottom and put in (-992) (if it were the smae example...)
There I hope you can understand that. It took me a while to get it myself.