First, choose a safe place to hang the lights. It's important to keep them away from power lines and other hazards. Next, make sure you have the right type of light strings for the tree you're decorating. String lights are the most common, but you can also use rope lights or icicle lights. Hang the lights on the tree one at a time, starting at the bottom and working your way up. Be sure to spread them out evenly and leave some space between each string. Finally, plug the lights in and admire your work!