I wanted the bag to be simple, straight and geometrical, so I drew the pattern (well, basically some squares and rectangles) myself. Assembling was also pretty easy. I first stitched quilt batting against the guitars fabric, to create a quilted effect. Then, I installed the zipper between the lining and the outside fabric, next stitched the rest of the outside (inside out) and then the lining (also inside out, but leaving a turning hole).
For the bottom, the lining, and the thingy around the straps (what's that called in English? I don't even know what it's called in Dutch) I used this cheap linen.