I've seen in hacks like Endless Earth that use warps that take you to different levels (and in Newer DS in some cases), and I'm wondering how you do that. Is there a specific guide to how that works?

you just set the warp to a number higher than the amount of areas in the level. for example setting the warp to go to area 4 in 1-2 is going to get you to 1-3 area 1. I have no idea how to make it go to a previous level though.
_________________________
Nothing to say, so jadnjkfmnjamnfjkldnajfnjkanfjdksan jsdnvj m.

i kinda know how to go back to previous levels

but theirs is a small chance it will freeze the game