Johnny Bravo, Slade 3 cannot make maps. It can only put new content such as sprites, code, etc. Into PWADS and IWADS, Doom Builder 2 is what you want for making levels. Slade 3 is good if you want to add new monsters and things. If you're looking for a tool for spriting, download GIMP. It's like photoshop, but it's free.
EDIT: Wait, I was wrong. Turns out Slade 3 CAN make new levels.