Firstly you must add a starting position for the Player. Right-click and select Info, then
info_player_start. See below

Then move it to where you want. Change to front view to make sure you are stood on the floor. It is a good idea to position slighty above the floor, because if the feet are inside the floor, the player will not be able to move.

Now save your level in the map/game directory of your Doom 3 directory. Do not save it in a folder like the real levels.

If you want a space in the name of your level, you must put _ instead.
e.g. My_doom_level

Now you must click on the BSP tab at the top, then click on the top four things one at a time. Wait for the console to stop before clicking on the next one.

Now save your level again.

Your level is now ready to be played! So press F2 to return to the game. Open the console and type

map game/XXXXX

Where X is, type the name of your level. Make sure you spell it correctly. See below

Press Enter and you will be able to play yor level!

You may have problems getting your level to work. If you do, email me
for help