Double knitting creates a reversible bag, with the inside being a negative image of the outside. The bag is knit at a tighter gauge than is usually called for in worsted weight yarn. This is for die security.

As you work it may be helpful to think in terms of “inside” and “outside”. Throughout the pattern all knit stitches make up the outside of the bag, while all purl stitches make up the inside of the bag.

Additional instructions on double knitting can be found online. KnittingHelp.com has a nice one that includes a pattern and video.