Measure all of your windows inside the casing. Subtract 1/4 inch from the length and 1/4 inch from the width of all of the measurements. Write down these numbers. These will be the dimensions you need to cut the plywood to.
Cut your plywood to the dimensions in step one. Write down on each piece of plywood the window it will be covering. You can also move it to just below the window as you cut each piece.
Place at least 2 Plylox clips onto each piece of plywood on opposite sides of the board. The legs will be pointing to the outside of the window. If your window is wider or longer than 24 inches, you will need more than 2 clips. The projected wind speed will also be a determining factor in the number of clips you need. See the resource link for a guide to the correct spacing between the clips. When in doubt, use more than you think you need.
Push each piece of plywood into its pre-determined window casing. The legs will be pointing outward, toward you. The clips will hold the plywood against the window casing.
Remove the clips by pushing the exposed leg inward to ease the tension. You can then pull the plywood off of the window.