Instructions
Press "Square" to open your Popit menu. Access the Popit menu while in your Pod or while playing a level.
Highlight the Reset Costume option and press the "X" button.
Highlight the Sonic skin from the Materials list and press the "X" button.
Press "R1" to access the Head menu. Highlight the Eyes list header and press the "Square" button. Highlight the Theme option and press the "X" button. The items in the Head category will now be categorized by theme. Scroll down to the "Sonic the Hedgehog" theme and select the nose and headdress items with the "X" button.
Press "R1" to access the Body menu. Highlight the Feet list header and press the "Square" button. Highlight the Theme option and press the "X" button. The items in the Body category will now be categorized by theme. Scroll down to the "Sonic the Hedgehog" theme and select the sneakers, gloves and spines items with the "X" button.
Highlight the Save Costume option and press the "X" button. You will now be able to access your Sonic Sack Boy at any time from the My Costumes menu.