You'll need DukeEdit. This tool can edit existing levels. Download on 3D Realms Site[legacy.3drealms.com].

Unzip the file to your game folder. Backup levels (WORLDAL ending with numbers or letters) if you plan to play the original levels later.

Run DUKEEDIT.EXE with DOSBox. It loads the first level. Get yourself familiar with what's there by moving around. Use the arrow keys. Press F1 for help on key definitions.