You find the exact measurements (fabric, interfacing, zippers etc.) in the official pattern at Craftsys:  

In some countries it's difficult to get the Pellon-Interfacing. Vilene G700 is the alternative.
I ignore the measurements for the Pellon/G700, I cut it exactly ½ inch shorter than the fabric-pieces.

The reason:
I sew with ¼ inch seam-allowance. That way I don't sew through the thick interfacing, only fabric to fabric.

The pieces B-F get the interfacing. In my bag the fabric for B-F is directional (the piano-keys). I put the letter of the piece on the back of the interfacing. Where the letter is, is 'up'.

It's easy to iron the interfacing to the fabric: take a damp piece of cloth, press for 10-15 seconds – that's it. It should like this afterwards:

The 2 pieces G and the 6 pieces H don't get an interfacing. These pieces are the inner part of the 4 zipper-bags, no interfacing necessary.

You need 4 side-pieces I, two pairs that mirror each other. The outside parts get the interfacing, the inside parts don't.
In my example: the outside will be blue, there is the interfacing. To get the right size, I cut the interfacing exactly the same size as the fabric-pieces. Than I draw a line with a pencil ¼ inch away from the edge and cut. After that I ironed the interfacing to the blue pieces.

The inner side will be the guitar-fabric. And it is directional, so I had to pay attention how to cut the fabric.