Last summer I made some super simple seat cushions out of some gorgeous appliquéd linen.
The simple pattern pieces, showing the seam allowance and fold back on the back pieces
Measuring the dimensions of the existing seat cushion for the front piece (and 1.5cm extra each side for the seam allowance) I cut the front piece for each cushion. For the back pieces, I cut 2 narrower rectangles measuring about 5 cm wider than half the width of the front piece so that they overlapped slightly. Rather than using a zip or button as a closure for the cushion covers, the overlapping back pieces created a pillowcase opening to slip the cushions into the covers.
I cut 4 long strips of fabric for each set of cushion ties and pressed and stitched each strip to hide the raw edges of fabric. I snipped the ends with zigzag scissors to prevent fraying and tacked into place on the front piece.