Sonic 2 Hacking Guide
Genecyst Savestate - Level Editing
Level editing it the most popular hobby by far. You can modify any level that is saved into RAM using a very simple format.
(8000-8FFF) - Floor layout
010C78-011277 (E800-EDFF) - Ring layout
The ring layout uses the same format as it does in the ROM itself. Check out this page for more details. The only difference here is that two zero's are added after the first four bytes.
Now you should probably know what values represent what floors. Please be aware that some objects look weird in EHZ, but look fine in HTZ.
002400-00D477 - Copy and replace for porting.
Yes, it's true! You can port levels between Sonic games. Copying the art is also recommended (012478-END).