O.K. Now on to part 2. I could have done this in 1 post, but it would have been extremely long, so I chose 3 posts.
Now, after you have sewn both of the sides try putting your cushion inside the "pocket" again. If it is too loose, you can make it a little tighter. If it is too tight, now is the time to rip out your seams and do it again (later on it will be much more difficult). Keep checking your work. I know it seems like a lot of extra steps, but it could save you from tears and a headache later.
Still with right sides together and your foam slab in the "pocket", grab one of the corners that you have sewn.
Pull it back so you have a fold (so to speak).
Put a pin in the crease of the fold. Do the same 3 steps to the other sewn corner.
Take a ruler or straight edge and line it up at the pin. (Mine is a quilter's ruler and it is great for this because it helps to keep your angles correct.)
Draw a straight line with chalk or disappearing ink pen (I lost mine so I used chalk). Then sew on the line drawn. Do the last 2 steps to both sides. This makes a box corner. It will make more sense when the project is completed.
Finishing rough edges. Starting where you sewed the "pocket" for your foam slab...
...the part that is not sewn fold down twice and iron.
Then pin and sew. Do this to both sides.
We are almost there so tune in tomorrow for the final post on 'Sewing a Porch Swing Cushion'.