Electrical Outlets and Switches
Using your utility knife, measure and mark where your outlet or switch hole is located. Choose to start your cut on the top or bottom of your panel (which ever applies to the location of your outlet) and cut out the rectangle. Test the fit before you peel off the backing. Make any adjustments necessary, place your panel and press firmly. Your cut should be covered by the outlet or switch plate after you place those back on.
Windows and doorways
If you have any windows or doorways, you might need to "rip" one or more of your panels. Ripping is when you have to cut a board horizontally along its width.
Lay your board up against the window or doorway and accurately mark what part of the panel you need to "cut out". Once you have done this, take your utility knife and make your cut along the line you drew. Test the panel on the wall before you remove the backing and make any adjustments to your cut necessary. Lastly, apply the piece to the wall and press firmly to adhere.