Measure your porch swing. Mine was 16 1/2 inches deep by 47 inches long. Get a piece of foam that size or bigger (I had to cut mine down with a serrated kitchen knife - pretty easy to do). Cover with batting.
Now for your fabric. If my fabric was 60 inches wide, I would just need to measure around the foam to find out the yardage needed (16 1/2" on top + 2" on front [thickness] + 16 1/2" on bottom + 2" in back = 37 inches). I will need more than that to allow for seam allowances and I want an overlap (so I can take my cover off to wash it). So I purchased 2 yards of outdoor fabric (I would rather have a little too much than not enough).
Now for the width of the fabric. My swing was 47 inches long and my foam was 47 inches long with a 2" thickness. I cut the fabric to 51" wide (from the 60" width fabric), that's 47" + 2" thick on the left + 2" thick on the right = 51 inches (that should leave plenty for seam allowance).
The edges that will be on the long side of the foam, turn under twice and iron, then pin and sew. This creates a "hem" to keep the fabric from unraveling later and it also gives it a much neater and more professional look.
This is my batting wrapped foam laying on the wrong side of the fabric.
I wrapped the fabric about 3/4 over the foam so I know where to pin it. Now measure how much the fabric overlaps the foam so you can turn your fabric wrong side out.
This is my fabric turned wrong side out.
This is my fabric turned wrong side out and pinned on the sides.
Here I placed the foam inside the "pocket" before sewing to make sure it was pinned right. Too much seam allowance and it wouldn't fit, too little seam allowance and it would be too big. It's a perfect fit. Now to the sewing machine again.
To be continued...