Cut the batting and the calico so that they are 120mm (4 3/4") bigger that the seat board all the way around.
We used a polyester batting about 6mm (1/4") thick.
This batting can be compressed to almost nothing around the sides of the seat board when pulled taut with the calico, so therefore it does not add much bulk to the sides.
The batting is quite strong, i.e. hard to pull apart.
Batting can often be obtained from upholstery stores, craft stores, fabric stores or curtain stores.
Place the calico on a work table.
Place the batting on top of the calico and then place the foam and seat board (upside down) centrally on top of the batting.
Begin at the middle of one of the long sides.
Stretch the calico and batting over the edge and staple to the underside of the seat board.
Next do the same to the opposing side and then to the two end sides.
Continue by stapling along one edge at a time, working from the middle towards the ends but do not staple any of the corners at this stage. Do one side, then the other side and then the two ends, trying to stretch the calico and batting with an even pressure all the way around.
Turn the seat over and have a look at your work. If you see any areas that appear to be pulled in too tightly, then simply pop out the staple there, adjust and re-staple. Likewise, if you see any areas that appear to be too loose then do the same.
Before finishing the corners, first cut away any excess batting and calico that may cause bulk.
Fold the corners over and fix with staples,
It is important not to accumulate too much mass at the corners, otherwise the drop-in seat might not drop-in as neatly as intended