I recovered this window seat cushion several months ago. It was originally yellow and not as thick but when I recovered it I decided to make it a little more comfy.
I originally bought 2 pieces of 1" foam at Walmart so I just bought two more pieces. It's now really comfortable. My grandbabies like to sit here and watch what's going on outside. This window faces South and the sun is great on a cold snowy day like today.
I cut two pieces of fabric the size of the seat with 1/4" seam allowance on each edge. In other words, I made each piece 1/2" larger than the foam. I then measured the circumference of the cushion and cut a strip of fabric wide enough for the sides.
If you don't have a continuous piece of fabric for the sides make sure to place your seam so it won't show. I cut two pieces so that my seams fall on the sides of the cushion up against the wall.
Then using your circumference measurement again, you need to cover a length of piping. The rule for this is to cut the fabric on the bias but I never do. I used a solid blue for the piping and just laid my piping along the length of the fabric to cover it. Then I trim it off.
I used to cover long lengths of piping before I started but then I had seams falling in the wrong places. So now I cut a piece of piping for each job and cover it. It looks a lot more professional.
You then lay your piece of piping a long the edge of your fabric - right side up - and using a piping foot or a zipper foot sewing closely along the piping. Sew your piping around the entire edge of your top and bottom piece of fabric.
Then sew the side piece of fabric to either the top or the bottom.
After you have your sides sewn on, add your bottom piece, leaving the entire back of your cushion cover open.
I would usually leave just an end open and slide my foam in but foam doesn't slide - at all You can use spray adhesive to make your pieces of foam stay together and put the foam cushion into your seat cover. You'll need to sew the back edge together by hand.
When I made my curtains I made the length about 4" shorter so that they would fall to the top of the cushion. Then add your pillows.
Have a wonderful week! It's a blizzard outside today here in Michigan with a high of 12 degrees. A good day to stay in, sit in a window seat and read a good book. And that's what I'm going to do!
Love and blessings!