Cut Your Shiplap
Once you have your measurements, you can cut your shiplap pieces to fit.
If you need to cut a piece around an electrical outlet, you can follow these steps to cut out your outlet box:
Using a full piece of shiplap, place it against your outlet, pushing it up to the chair rail. Mark the top and bottom of the outlet on the side of your piece.
Take a scrap piece of shiplap and tuck it into the groove directly above your outlet. Mark the side and left side of your outlet on your scrap piece. Then, use your scrap piece to create a template on your shiplap piece.