Instructions were simple. I used the correct version of the ROM as well, and for whatever reason the patching process makes the checksum go bad and the ROM becomes corrupted.

There are three possibilities I can think of:

The ROM is bad or somehow incompatible with the patch. It would have to be something that allows the initial ROM to function normally and only screws up when patched.

The patch is bad (or incompatible).

The patching program is not working properly or isn't meant to work with this specific kind of patching.

I was using Lunar IPS. I wonder what the next step would be? I already tried a couple different ROM files and neither patches correctly, and none of the patches I tried worked (Lunar IPS said they were successful but then my emulator wouldn't run the patched ROM). I'm thinking this might have something to do with the patching program...?