After you beat the game, on the file select screen, you can now select whichever stage you want on your cleared file.
Choose Studiopolis, and follow this path, you'll reach the giant special stage ring in about 10 seconds:
https://imgur.com/16ILlFWWin or lose, once you exit the special stage, just pause and restart the level.
Repeat until you get all 7 emeralds.
None of the stages are TOO difficult, just practicing them a couple times to learn the layouts can get you through. But here's a video showing them all if you want: