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Trim the foam using a good sharp pair of scissors to fit the area you want to upholster. Then use the scissors to cut a 45-degree beveled edge on the foam. This will give the cushion a clean, rounded edge when the fabric is stretched over it.
Apply spray adhesive on one side of the foam and on one side of the plywood backing board. Center the foam on the board, being careful that it is centered. Let the adhesive cure.
Stretch a layer of batting around the foam and backing board, smoothing it out as it’s laid on to the foam. Once you have a good fit, carefully lift and apply spray adhesive to the underside of the batting and the top of the foam. Smooth it out and allow the adhesive to cure.
Now stretch your muslin over the backing board, foam and batting. Leave enough extra around the edges to wrap the muslin all the way around the board. Be sure to pull it tight and secure it on to the back side of the backing board with a staple gun. First staple the center of the long edge, then staple the opposite side in the center putting enough counter pressure to keep a firm fit. Then staple the centers of the short edges the same way. Finally keep stapling through the batting and muslin all the way around the perimeter of the back side, trimming off excess muslin as you go. Be sure to keep a tight fit and work out any wrinkles as you staple. A good guide is to put in one staple every inch, but use as many as needed to get a good, tight, wrinkle-free fit. Lightly tap in protruding staples with a hammer.
Wrap your "show" fabric over the muslin. It should go on easily and smoothly now that the muslin is wrinkle free. Staple it to the back of the board the same way as the muslin (Image 1). If your show fabric has a pattern, make sure you line up the pattern straight across your edges.
Finally, add the upholstered cushion to the storage bench and nail front trim into place (Image 2).