Activate both Test and Test2, then purposely reach Game Over in your current file. The Debug Mode should now appear when you return to the Load Select menu.
On Floor 2 you will eventually find a room with a Scroll 2 beside a skeleton (after you find the Yellow Orb). Shatter the columns to the right of the scroll to reveal a switch that opens another secret passage where you can find a useful item called the Sextant. This device allows you to instantly warp back to any teleportation room you have discovered, anywhere in the game.
Reach a room in Level 4 you will find a mirror reflecting images of your character and some zombies. Defeat the zombies, then jump on a box in the center of the room and push against the mirror to reach a secret room containing a pendant.
Activate Test Mode as described in the "Test Mode" tip and enter the Test menu. Press Up, A, Right, A, Left, A, Down, A to activate Test2 mode where you can view rooms, error and system logs, and more.
Pause the game, enter the Mode menu and uncheck all items. Then press Down 8 times. This unlocks the Test option where you can access the sound test menu, view sprites and dialogue text, and more.