Instructions
Move your Sackboy around with the left thumbstick. Press "X" to jump -- the longer you hold down the "X" button before releasing it, the higher your Sackboy will jump.
Aim with the right thumbstick. There is no targeting reticule, so you must take notice of where the rocket launcher is aiming. To make sure your rocket flies as straight as possible, stop moving as soon as you have your target in sight.
Tap "R1" to fire your rocket. If you stop moving the rocket launcher and your Sackboy before firing, the rocket will fire straight at its target.