Unlock Sonic Mania stage select and debug mode:
http://www.nintendolife.com/news/2017/08/guide_unlocking_sonic_manias_debug_mode_and_stage_select
http://www.nintendolife.com/news/2017/08/guide_unlocking_sonic_manias_debug_mode_and_stage_select
In order to access this mode you simply need to hold the B and Y buttons down on your controller of choice on the title screen before the 'Press Any Button' prompt appears. Then once it has, continue to hold these two buttons and press the Plus button. You'll be catapulted into the stage select menu where you can choose to play any stage the game has to offer.
Once you're in a stage if you press the X button you'll enter debug mode, which allows you to fly around freely and place various objects such as Rings, enemies, and Item Boxes. Objects can be cycled through with the B button, placed with the Y button, the X button returns you back to being Sonic, and the A button cycles through the different Item Box variations.
