Wipe the plywood down with a tack cloth. This will remove any small particles of dirt that can get in the way of the scoring process.
Start your first vertical score line. Lay down your yardstick near the edge of the plywood. Place the utility knife at the top of the stick and bring it down. Do not overapply pressure or you may snap the blade. Move the yardstick as necessary until you've gone the length of the board.
Continue your vertical scoring. Move your yardstick over about 3 inches and create another line. Keep doing this until you reach the other end of the plywood.
Score the plywood horizontally. Turn your yardstick around, start at the edge of the board and begin your horizontal lines. Move the yardstick over 3 inches and continue making horizontal lines until you reach the end of the board.