I fussy cut the 4" wide front gusset strip so that it was centered on a length of rope and long enough to wrap around to the sides a bit. I cut the side gussets out of the plain white sections since there was not enough rope to go around and the sides would not show anyway.
Here is a test run of the assembly laid out on our bed before having a go at assembling it.
First I joined the gussets strips in a continuous ring and adjusted for snugness on the foam. Then I marked where the corner positions were. When I googled how to sew a gusset cushion this was recommended as the best sequence.
With right sides of the fabric together, I sewed the first corner to the rectangular top with no problem at all. I had to rip out the second corner three times before I could get it to lay right without puckers or tucks. With sixteen corners to deal with between the two seat cushions, I needed a better method. I finally gave in and ran a line of stitching along the seam line of the gusset at each corner and snipped inward, just short of the stitching line. I had not wanted to weaken the corner in this way, but it made the construction world's easier to stretch the gusset around each corner of the top rather than to try to ease the top into a 90° angle. It was slow going, but the remaining fourteen corners went together smoothly once that little slit was there for wiggle room. And there really was no strain put on the corners in that direction anyway. That part of the seam allowance got folded under.
Here the cushion top is attached but not yet the cushion bottom. I great tip I learned from googling was to remember to unzip the zipper before putting the bottom on. Otherwise I would have constructed a fully zipped inside out cover with no way to reach in and turn it right side out. Duh!
I had to do some tweaking by nipping in the edge line seam where a bit of selvage writing peeked out. This was no problem. It just gave the cushion a snugger fit. Had it been an issue, my solution would have been to shave a bevel along the edge of the foam. That was not necessary since the foam was forgiving enough.
However, wrestling the foam into the completed cover was quite challenging. Even though the foam is squishy, it grabs at the fabric and nothing slides. Putting on non-stretch jeans a size too small would have been an easier undertaking. Mission accomplished though. I will go back and adjust the directional lay of some of the seam allowances later but for now both cushions are done and deployed!