First, let me explain what DEBUG Mode is. DEBUG mode allows you to basically place the same things wherever you want in the same levels. You can also find rings, spikes, and
robots that you can encounter in that level. But what makes DEBUG Mode really special is the Sonic Mega Collection. An SMVX is a monitor/screen. When you jump on it, you immediately become hypersonic! (a stronger version of Super Sonic) However, before you can do that, you also have the permission to choose a level. Now, I'll show you how to do it.
Start a game without saving. In Angel Island ACT 1, you need to hold on to one of the vines. Hang on without jumping off, then move the thumbstick in the following way:
LEFT, LEFT, LEFT, RIGHT, RIGHT, RIGHT, UP, UP, UP
Choose a level and there you go! You have now activated DEBUG mode and can select levels yourself!