Cut out a piece of fabric that is a bit wider than the cushion you are covering and and long enough to go all the way round the cushion and probably about half of that again.
On the wrong side of the fabric turn the short ends down 1cm and then over again 1cm. Do this to both short ends and pin them or press them to hold it in place. Then to secure sew along both edges.
Still with the wrong sides facing you, fold the fabric over till the short ends overlap and it is the correct size to cover your cushion. Then pin the open ends and sew them together, I then usually zip zag over the seam to make the seams extra secure and prevent fraying.
Turn the cushion cover so the right side is now facing you.
Take a plastic bag and cover the cushion. Tie the ends of the bag and sellotape it so that its completely covered:
Put your newly plastic covered cushion inside your beautiful new cushion cover and your are finished!!