One easy (easier) way of doing them:
make a piece of fabric that's 2.5 times the length of your cushion pad and the same size plus a good seam allowance wide (think squashed, though - the easiest way to measure is to grab the sewn end and stretch it and measure that. then add a couple of cm each side)
make a hem on EACH END of the length.
fold it so that (0.5 times part of the fabric) is folded down wrong sides together and the (2 x the length of the fabric) is fold evenly right sides together.
you can pin if you need to.
sew all the way down both sides of the fabric.
Turn the right way round (please remove pins first)
You?ve made something like a pillow (it?s not perfect because you might have exposed edges at the very top, but you can do other stuff to neaten it up, but I can?t explain it)
stuff your cushion pad in, and tuck the (0.5length) part over the cushion?s exposed edge.