Now comes the big moment: is piece A long and wide enough?

I had cut it huge for the guitar bag. And it was long enough. And it was way to wide – my fault. After step 4 (sewing the pockets) I had to trim the pockets a lot, I was kind of sloppy when I sewed them.

I started trimming one side of piece A and fixed it with Wonderclips to the bag, then sewed it (on the fancy seam again).

I carefully measured and cut the other side of piece A along the bag with the rollcutter.

And as usual fixed it with Wonderclips and sewed.