Snagged a random possible chair restoration project (free) down the block. Tough one, no existing cushions. But I still have foam and vinyl for a chair or two here. #furniturerestoration
@ai6yr
1-1½" foam is fine thickness for dining chairs. Or you could go old school and use felted cotton batting.
The pattern is easy. Use that bottom support and cut padding 1" larger all around so you dont feel the hard edge of the support.
For the covering fabric, add the thickness of the pad and about 3" all around for when you wrap and staple the fabric to the support.
Remember to enlist a buddy or child for the extra hands. One to pull the fabric with pliers, one to weild the staple gun.
The brighter coloration around the edges is probably where the fabric wrap covered the wood, and kept it less oxidized and dried out. Look for staple holes.