First, I measured the width and depth of each seat to see how big the cushions should be. The cushion filling I bought was 4″ tall, so I knew that number would be the height of my cushion.
To figure out the fabric dimensions for the cushion panels, take the width and depth measurements and add 1″ to both numbers to allow for a 1/2″ seam allowance all around. These measurements are for the top and bottom panels of the cushion. To determine the side panel length, add the original measurements (the numbers before the additional inches for seam allowance) of all four sides of the large panel together. Add 1″ onto that big number to allow for a 1/2″ seam allowance. The height of your super-long side panel is the height of the cushion filling with 1″ added for a 1/2″ seam allowance.
Take these dimensions and cut your top and bottom panel and your long side panel accordingly.
Use fabric scissors to round the edge of your fabric corners on your two larger panels. To connect your long side panel strip and make one big continuous loop around the edge of your cushion, fold the panel in half (with the right sides facing each other), and sew 1/2″ from the edge to connect the ends. Line up the edges of your side panel loop of fabric with the edge of the top panel (with the right sides together), and pin the edge in place.
To make the corners easier to sew, add some “V”-shaped notches around each corner.
Use your machine to sew all the way around the edges with a 1/2″ seam allowance.
Pin the bottom panel to the side panel in the same manner as the top panel, and make notches on all the corners.
Before you sew all the way around the bottom panel, you’ll want to add a zipper at one of the ends so you can insert your cushion filling. Place the zipper face down onto the right side of the fabric at one of the ends of your bottom panel. Pin the zipper in place so that the zipper will be installed 1/2″ from the edge.
Use a zipper foot on your machine to sew the right side of the zipper in place.
Repeat the process of pinning and sewing the other side of the zipper so that you have a functional zipper installed. Now you can sew all the way around the rest of the bottom panel (starting at the end of the zipper and stopping when you reach the beginning of it). Make sure to leave your zipper open a few inches before you sew all the way around so you can get back in later! Flip your cushion cover right side out, and you’re ready to add the filling!
Use a ruler and a marker to measure the appropriate width of your cushion filling.* If you aren’t able to buy a cushion long enough that’s in one piece, you just use as many pieces as needed to fill the cushion length. I would highly suggest making the width and length of the insert a little bigger than your actual cover dimensions (maybe add an inch to both). This way the insert will fit snugly into the cover and you will have a full-looking cushion.
Now, I’ve heard that some people use jigsaws or even electric turkey carving knives to cut foam and filling, but the thing that worked the best for me was actually a plain old handsaw. Weird, right? The saw was able to do most of the cutting pretty easily, but I did use some fabric scissors to clean up the edge a bit. Once my filling was cut to size, we folded the filling in half like a taco shell and inserted it into the cushion cover through the zipper opening. Use your waterproof spray to seal your fabric, and you’re done!