To put Christmas lights in trees, you'll need to follow these steps: First, determine the layout of your lights. You can do this by measuring the length and width of the tree and deciding where you want the lights to go. Next, gather all the materials you'll need, including the lights, extension cords, and a ladder if necessary. Once you have everything, plug in the lights and carefully wrap them around the tree branches. Be sure to secure them tightly so they don't fall off. Finally, plug the lights into the extension cord and turn them on to see your festive Christmas tree!