This is the easiest method of making a cushion cover as you do not need to set in a zip or sew buttonholes or any other fasteners to close the cover.
Measure the width and length of the cushion. Cut a rectangle that measures twice the width of the cushion plus 20cm for the opening section, and the length of the cushion plus an additional 2cm for the seam allowances.
On one short side, fold a double hem 1cm wide. Machine-stitch in place. This is for the inner opening at the centre back. On the opposite side, fold a 3cm double hem. Machine-stitch in place. This is the outer opening and forms a hem and facing.
With the wrong side of the fabric uppermost, take the side for the inner opening and fold it over so it sits in the centre. In the same way take the other edge over so it overlaps the first edge. Adjust the overlap until the fabric measures the actual width of the cushion.
Pin and tack the layers of fabric together so they do not slip while you are sewing the seam. Machine-stitch the top and bottom seam, working through all the layers of the fabric. Work a second row of stitches at the overlap to reinforce the first row. Remove the pins and the tacking stitches.
Turn the cushion cover to the right side through the centre opening. Press the cover and then insert the cushion pad through the opening.