The sides of my quilted cushion are a 5"thick foam.
 

How do I make sides of a cushion cover to go over a 5" thick foam? Somehow I've managed to get this old without ever doing this.

I was going to try to explain....but I'm not terribly good at it, so here's a reasonably straightforward method
http://www.ehow.com/how_4798593_sew-box-cushion.html
This is for a piped cover without a zipper.....you could leave out the piping if you wanted.

If you're sewing cushions, etc - allow for 1/2" seams and don't forget to allow for some ease also - just 1/4" is enough...

I would for sure use the piping, it will give a nice finished look. Just my opinion.

And use a zipper foot if you are using piping.

Excellent directions available in the link. One more suggestion. Cover your foam cushion with the thin plastic bag from the dry cleaners (like they put over coats or dresses). It will be easier to slide the new cover over and then you can just rip it out before you sew the seam opening closed.

Pure genius....thanks for the tip:)

Two comments on the post 1) 'The ring should open at the corners' I assume she means you have the clip the ring at the corner in order to make a turn. 2) Be sure that your ring, as she calls it, starts at the same place on the top and the bottom. If the seam is in the center of one side, be sure it's exactly in the center on the other. And for good measure, if you're using older foam, a wrap of polyester batting around the foam refreshes the look.