All credit goes to a user named Signess, I found his list on the gamefaqs forum. Finding the red star rings earns you artworks, music and also new skills you can unlock.
The link: http://www.gamefaqs.com/boards/627224-sonic-generations/6084...
(Just browse through the pages untill you find the stage you need).
Also, Youtube user SadFaceGaming11 has uploaded a video guide for all the stages. They're very good and will really help if you can't be bothered to use a text guide (like me). Good luck!