In the game, get to the title screen and push start to see the selections. In Mednafen, push Alt+D, then push Alt+3. In the memory editor window, push G and type in 10EB. At the cursor position, push Insert and type in 03. Push Alt+D to close the window. Now push A in the game to proceed (it doesn't matter where the cursor is).
tldr; Memory editor, go to 10EB, type in 03.
You should now be at the debug menu. It has a sprite viewer which I'm really excited for. There are lots of cool features in there you should look around for. In the map viewer, I found this to be interesting.
This is really interesting, so have fun with it! Here is a video showing everything in it.