First go to Angel Island Zone and, while on a vine, press Left, Left, Left, Right, Right, Right, Up, Up, Up. If it has been entered correctly, you will here a chime. Then press Start then A. At the Title Screen, scroll down to Sound Test and press Start. At the Sound Test screen, go to Mushroom Hill Zone and press Start. While on a ratchet machine, enter the same code you entered in Angel Island. You should here a chime. Again, press Start then A. At the Tiltle Screen again scroll down to Sound Test. At the Sound Test Screen select a zone while holding A. Hold until the name dissapears. You can check if you're in Debug Mode by pressing B. Your score should turn into hexadecimal characters. The controls are these:

With Debug Mode on:

A- change sprite B- disable Debug Mode C- add sprite
With Debug Mode off:

reverse gravity
enable Debug Mode
jump