I removed the padded seats from two chairs I scavenged from the neighborhood "loose trash." Rather than purchase new foam (it's expensive), I removed the foam from the existing chair seats to save for later. Then, I removed the wood seat portion and front legs of the chair from the back and back legs.
To build the bench seat, I cut plywood and siding according to the width of the chairs and the length I wanted. I screwed the siding into the plywood to make a wooden box.
Next, I painted the entire piece with primer and then exterior paint.
I hot glued the two foam pieces together to fit onto the size of the plywood that I was going to use for the bench seat and that would also fit onto the top of the frame. Then, I used the staple gun to attach the fabric to the plywood. Then, I screwed the upholstered bench seat to the top of the frame.