If your tree isn’t pre-lit, start by stringing your lights. Make sure to plug each strand in to check that they’re functional before you start.
Work from the top down. Start at the trunk, circling lights around a branch out to the tip and then back in before moving on to the next branch. Aim for about 100 lights or more for every foot-and-a-half of tree. And if part of your tree is facing a wall, don’t bother lighting that section — no one will ever know!