Instructions
Open create mode from LittleBigPlanet's main menu.
Press "Square" to open the tool's menu and select the Emitter.
Press "Square" to open the tweak menu for the Emitter. Press "X" with the first circle icon in the tweak menu selected to choose which item you want to launch across your level.
Use "X" and the left control stick to change the other options for your Emitter. The first icon under Item Settings controls how fast the object will launch out of the Emitter, and the first icon under Timing controls how often the object will be launched. Be sure to set it above zero so that the launcher will automatically reset and continue to launch your item again and again.
Use the left analog stick on your controller to position your emitter where you want it in your level. Then press "X" to set its location.