wYou need to salvage/get a spring base and then cover it with wadding. This package is much larger than the cushion cover that will go over it. To overcome this get some slip film and wrap the cushion and the end of your vacuum cleaner hose up and then vacuum it down (just like a storage bag) until it does not go any further and then insert the lot in the cover. Release the vacuum hose and then stitch the cushion up. This is a very simple explanation but with experimentation will get the job done. Hope it helps.

PS An excellent reference for the handyperson upholsterer is "Upholstery A Complete Course" by David James.