Fit the PVC pipe insulation to the desk edges, allowing a slight overlap at the corners. Cut the insulation to fit with the utility knife.
Remove the PVC pipe insulation and apply the double stick tape to the edges of the desk, leaving the corners untaped for now. Slowly, install the insulation again, pressing it down flat as you go.
Cut out any corners by using your utility knife. Make a V cut and fold back the insulation. It should fit pretty snugly, as long as you make the cut right at the corner. Install the double-stick tape at the corners and press the insulation down in place. Trim off any excess material that goes past the desk edge. Or, fold it around a bit and tape in place.