Measure the width of each riser and then mark the center where the fixture will be placed. Remove the screws and front faceplate from the fixture.
Hold the fixture up into its approximate final location and measure down about a half-inch below the tread. This is where the fixture will sit. Use a level to make sure the fixture sits level underneath the step.
Make a pencil mark across the top edge of the fixture.Measure the inside portion of the fixture that gets recessed into the step riser. This determines how big of a hole will need to be cut.
There are many factors to consider when deciding how many step lights you need for your stairs. You should think about the length of the staircase, the height and width of each stair, as well as safety concerns like where people will be walking during nighttime hours.
If you have a short set of steps with only two or three risers, you may only need one or two step lights. If the steps are wider, then your lighting should be more evenly distributed across them to ensure safety and visibility.