Cut a piece of 1/2" plywood to the exact same size as the top of your bench.
Lay your piece of plywood on top of your bench foam, lining it up on two sides at a corner. Use a pen or marker to mark the other two sides.
Cut the foam along the marks so that the foam is the same size as the plywood.
Lay the foam on top of the plywood, and lay the batting over the foam. Cut the batting to size, leaving a few inches overhanging the bottom of the plywood on all four sides.
Build the cushion
With a drop cloth underneath the plywood, coat the entire top surface of the plywood with spray adhesive. Place the 3" bench foam on top of the plywood, line up all the edges, and gently press down to attach the foam to the plywood.
Lay the batting on top of the foam and center it so the overhang is even on all sides. Use spray adhesive to attach the batting to the foam ONLY on the 3" tall sides of the foam; do not use spray adhesive on the top surface of the foam. Press along the sides of the foam all the way to the corners to ensure the batting is firmly attached, leaving a little pucker of batting at each corner.
Trim the batting along each side so that it is flush with the bottom of the plywood.
To trim the batting at the corners, smooth the batting straight along the sides until you reach a corner, then pull the pucker of extra batting straight out and away from the cushion. Cut straight down, through the batting, right at the corner of the foam.
Smooth the batting along the sides of the foam and into the corners so the corners aren't wrinkled.
Attach the fabric
Lay your upholstery fabric on top of the batting and line it up so that any patterns are straight on the cushion. Cut the fabric to size, leaving 3-5" of overhang past the bottom of the plywood.
Turn the entire cushion over (including the fabric) so that the plywood side is facing up. Gently pull the upholstery fabric straight back towards the center of the plywood (perpendicular to the edge of the plywood), then staple it in place using a staple gun.
Continue stapling the fabric to the bottom of the cushion on all four sides, making sure there aren't any large gaps between staples. Stop about 4" from the corners to leave room for pleating the fabric at the corners.
Upholster the corners of the cushion
To upholster the corners, find the middle of the loose fabric at the corner and pull it up and over the corner of the plywood at a 45-degree angle. Staple the fabric once to hold it in place.
Starting on one side of the corner, fold the "wing" of loose fabric up onto the plywood and form a vertical pleat running up and down the corner (like wrapping a present). Staple the fabric once right near the corner to hold it in place.
Repeat the same process with the fabric on the other side, pleating it so that it too has a straight crease running vertically along the corner. Staple once to hold it in place.
Cut away excess fabric to remove some of the bulk so your cushion can lay flat on your bench. Leave 1" to 2".
Once the excess fabric is gone, add a bunch more staples to hold everything in place. If any fabric sticks out, fold it onto the back of the plywood (as flat as possible) and staple it. Repeat for the other three corners, then go back around all four edges of the cushion and add staples to fill in any gaps.
How to remove staples, if needed
If you need to remove a staple, carefully slide the front of a staple puller under the staple, gently wiggling it side to side, until the staple pops up.
Install the cushion
Turn the cushion over and position it on top of your bench. Drive screws from underneath the bench up into the plywood of the cushion to secure the cushion to the bench.
Using a hot glue gun, attach the upholstery trim to the very bottom of the cushion to hide the gap between the cushion and bench.
If you prefer to wrap the batting all the way around the bottom edge of the plywood and staple it rather than using spray adhesive, you can totally do that! Just follow the instructions in the "Attach the fabric" section when attaching the batting, then repeat those instructions a second time to attach the fabric. Don't try to do the fabric and batting at the same time.
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