To hang Christmas lights outdoors, you need to ensure that the lights are rated for outdoor use and that the cords are waterproof. First, start by measuring the area you want to decorate and determine how many strands of lights you will need. Then, use a ladder to reach the areas you want to decorate, and use hooks or nails to securely attach the lights to the roof or other outdoor surfaces. Finally, make sure the lights are connected to a GFCI outlet to prevent electrical shocks.