To make this cushion you will need a cushion filler pad and some fabric. I made it using two different vintage fabrics. You will also need the zipper. Measure your filler pad across from seem to seem and add 2.5cm (1") to it. This will be your measurement for cutting the square of fabric for the top of the cushion. For example your pad is 22" from seem to seem, so your fabric square will measure 23" x 23"cm on each side.
The zipper will be in the middle of the back of the cushion so you need to cut 2 pieces of fabric for the back. To get the measurements for the back pieces, add 5cm (2") to the front piece measurement and divide it by 2 (23"+2" = 25"; 25":2 = 12.5"). Your 2 back pieces will measure 12.5" x 23" each.
The zipper should be about 5cm (2") or more shorter than the measurement from seem to seem of the filler pad.
Machine sew one of the long edges in both back pieces with zigzag stitch. To make it easier to sew the zipper in, first fold and iron the edges on one side. Fold one piece 1.5cm (0.5") from the edge of the fabric and the other 2.5cm (1") from the edge.
Take the piece with narrower fold and using the pins secure the zipper in place, starting about 1.5" from top edge, making sure the zipper is positioned the same distance away from top and bottom edges of the fabric.
Change the foot in your sewing machine to a zipper foot and sew in one side the zipper.
Position the other piece of fabric (with wider fold) over the zipper so that the two fabric pieces overlap by about 1 cm (less that half inch) and secure with pins. Mark 2 places where the soft zipper ends are and secure with pins like in the photo:
Start sewing in the other side of the zipper. When you get to the marker across, sew this place 3 times, to make the stitch hold the zipper strongly in place. Continue sewing along the pins, removing them just before you get to them with machine needle.
When you reach the end side of the zipper, repeat the triple stitch action where the pin marker is and continue sewing to the end.
This is what the zipper should look like:
Place the back piece of cushion cover on top of the front piece with good sides facing each another. Cut away any uneven edges and secure 2 pieces together with pins. Make sure you zipper is unzipped enough for you to insert your hand inside. You will need it to fold the cushion cover inside out later on.
Change the foot in your sewing machine to standard one. Sew 2 pieces of cushion cover together 1.5cm (over 0.5") from the edges and finish off with zigzag stitch.
You can also fold the corners and sew them across, but it is optional.
Fold the cushion cover inside out and iron the stitches. Insert the filler pad. Your cushion is ready.