All you need is Doombuilder, preferably one of the latest dev builds of
GZDoombuilder, and of course the Doom2 iwad. Should work out of the box; if not, you might need to install SlimDX, too. While you're at it, grab some source port for for easier playing/testing, e.g
Zdoom or prBoom.
You can load one of the original maps to see how things work. Basically, what you do is draw the layout of the level and compartmentalize everything into sectors (DB does this automatically) which then can be assigned different floor/ceilings heights. There's a 3D mode for texture alignment, and so on.