Measure the width and height of the cover. The height is important because this is the tightest fit on the cushion.
Cut the cushion to fit with hedge trimmers. These work like large scissors and will give you a very accurate cut with straight lines.
Open the cover as wide as possible and place it over the bottom end of the cushion. This will become the front of the cushion. Fit the ends and slowly pull up the cover. Pull equally on both sides to give the cushion a firm fit. Zip the end of the cushion.