To make posters, simply select the Poster Maker from the main menu. Backgrounds and stickers can be unlocked by playing the music minigame (either through the main menu or by tapping the locked content in the Poster Maker).
Drag items from the Poster Maker tray onto the canvas to use them. Once stickers are on the canvas, they can be scaled by pinching two fingers, rotated by twisting your fingers, and moved by dragging them.
To remove stickers, just drag them back into the tray. Once you're done with the poster, tap the confirmation button, and you can save it!