How to lock level onto a 2D plane?

[nambona890]

I'm very novice in SM64 hacking, and I've only made 1 hack in the past (don't ask what it was lol). A thing that I've wanted to try out is having a 2D plane type level, like in Super Mario 64 Run, and the 1-1 implementation. What is the way to do this? Thanks!