This is a very easy project. Just follow these simple steps:
1- Measure seat (or backrest) of the chair and cut foam accordingly.
2- Use foam shape to trace pattern on fabric (two layers).
3- Add seam allowance, don't forget to take into account the thickness of the foam.
4- Cut fabric (duh, I know. But hey, it's easy and this review needs to be 100 words).
5- "Sandwich" the foam between the two pieces of fabric, right side facing out.
6- Pin in place.
7- Sew all around.
8- Trim seam allowance.
9- Hide raw edges with bias tape.
For the second cushion, I saved some time by sewing one side of the bias tape at the same time as sewing step 7. It saved a seam.
The only (small) difficulty of this project is dealing with the third dimension that is the foam. I used a zipper foot to make sure the seam was as close as possible to the foam.