If you make the fabric-dish:
Attach the 2nd magnetic snap on the inner part of piece I (the one on the right side of the bag).
The pattern says where to place it..
(Sorry, no dish for the guitar-bag, so no photos. But it would be the guitar fabric for this bag).

Put both pairs right sides together and sew with 1/4 inch seamallowance – but leave the long edge at the bottom open.
(Ignore the interface if it's uneven and sew with ¼ inch anyway.)

Along the seam: cut a V into the 3 corners, cut small cuts along the curve.
Turn the piece inside out.

Topstitch as close as possible to the edge but only on the seams. Leave the long edge open.

Turn again. And cut away carefully with scissors all the redundant fabric. Turn again.

Topstitch again, but this time ¼ inch from the edge of the fabric (not from the former topstitching).
And this time topstitch around the whole piece.

The long edge gets the topstitching, even if it's open and not serged.
That doesn't matter in this case. This long fancy seam serves as an orientation line during the following steps.
This seam is one of the reasons why I prefer to sew this bag with Superior SoFine/Bottom Line.
Both threads are extremly thin. During the following steps there will be so many seams on top of the fancy seam – it's good to not make them thicker than necessary!

Put one piece on top of the pattern, the other parallel below. Mark the 4 lines with a chalk marker.

Iron the folds (blue will be the outside, so I folded around the guitars and ironed).