This is always fun. We turn the cushion outside out and we inspect the corners. If we find a flaw, we fix it now before putting the cushion in. The cushion will add pressure to all the fabric.
When turning the cushion outside out, you push the corners out. At this point, it is good to have already clipped the excess fabric from the corners.
Of course, everything looks good at this point. It isn't until the foam is inserted that you realize how well you did.
Since I just thought of it, let me provide a word on "batting."
Basically, the foam has "batting" which is like the stuffing of a pillow put all around it. This will make the cushion cover smaller than the foam and batting which are inserted into the cushion. This provides stiffness.
So, we size the cover to exactly match the foam. We add batting around the foam (already added in our case since we reused foam+batting). The batting adds to cushion comfort and it smooths the edges of the foam so that the edges are not visible through the fabric. The batting is glued to the foam. There are videos on Sailrite about batting.