First create or edit the Level (.ld) file using the Map Editor (M.exe). Then use the helper batch file mkpdb.bat to insert the Level into a Game Database (.pdb).
To add a Level to a Game Database:
mkpdb -pdb <database> -ld <level> -size <16 | 24>
Example:
mkpdb -pdb htdata816.pdb -ld grassy_test.ld -size 16