Instructions
Open your Sackboy's Popit menu by pressing the "Square" button on the controller. This will open a small window that contains all the tools you need to create and edit your level. The "Lasso" tool allows you to pick up and move each object, and the parts for building your level can be found in the "Objects" menu.
Open the "Objects" menu and navigate to your background pieces. Select one that's appropriate to the theme of your level. If there are none that suit your design, try playing some similar levels and seeing if you can collect any additional background pieces. For instance, if your level has an African theme, try playing through the African levels in an attempt to gain some more appropriate backgrounds.
Select the background piece and drop it into the level in the desired spot. Return to Sackboy's Popit menu and select "Score Bubbles" from the list of available objects. A bubble will appear at the end of the lasso. Position it in front of the background piece and hold the "X" button to glue the bubble to the background piece.