This is how the seat should look with the calico stapled in place.
The seat is now ready for the cover fabric.
This can be cut and put on in the same manner as the calico, but take special care with the folds at the corners since this is the final cover and the fabric that will be seen.
Line the underside of the seat.
This is just a matter of stapling a piece of calico with folded edges to the underside of the seat, basically to make the whole underside look tidy.
The seat should just drop into its rightful place in the bench.
Now just add 5 or 6 screws up through the seat support strips into the underside of the seat board and it's all finished.