Okay so I found this old chair in the attic, and the fabric from the cushion was all worn out and gross, so I thought I would make it nice again :)
Okay so take off the old chair cover. Cut through the old elastic and iron it flat.
Trace onto the new fabric and cut out.
Make new casing for elastic and pull the elastic through with a safety pin.
Pull it over the cushion and voila your done :D