So make lets make the first sector. To change use Tab to change the second number. They must point inwards into the sector because when you make lines into a sector, the editor will add a texture on that right side of the line with the small line. This makes the lines into a new sector. The level is almost ready to try.
Press insert for the start position somewhere in the big room.
Then click on the item and press Enter to modify the thing. Change it to a player 1 start position. Now you should be able to try the thing out. Now save you level and exit. There will be some messages concerning Multiplayer starts and deathmatch level start but we don't need them yet.