Step 9: Make folds and stitch down
The A line on the fabric and vinyl is probably covered, so use the edge of the quarter inch ribbon as your A line.
It is not as important that the top B meets A because this is the top of the pocket and it will not be sewn down.
The top pocket has only a tiny fold so it difficult to pin down. Just force it and do not worry if it is not perfect.
Sew bottom of pocket down using 1/4 inch seam. Paper helps here too.