Measure the width of the area where you countertop will go. Write down this measurement. Measure the length of the countertop. If necessary, trim the length of the countertop down to size by with an electric saw.
Run a stud finder along the wall where the countertop will be attached. Mark the areas where the studs are with a piece of chalk.
Turn the countertop over and drill one side of two L-brackets into the bottom of the countertop, spacing them 1 to 2 feet apart.
Prop the countertop up against the wall, adjusting it so the L-brackets rest against an area where the studs are. Drill the other side of the L-brackets directly to the wall.