Summarize:

This forms a roll of cloth, hiding the raw edges of both layers.
Insert the bag interior into the bag exterior. Clip the inner layer so that it’s 2 inches (5 cm) higher than the edge of your laptop (or box, in this case). Clip the outer layer two inches (5 cm) longer than the inner layer. Fold the outer layer twice – once in and over itself and once again over the inner layer – and pin for sewing.