The bag is made using Tunisian Entrelac crochet.
Tunisian crochet is a different method of crochet (similar to knitting) consisting of a “forward pass” (working right to left and picking up all the stitches in a row) and the “return pass” (working back, left to right, to remove the stitches from the hook). ‘Entrelac’ is the technique of working one shape at a time.
This pattern is Intermediate as there are mid-row colour changes, however full instructions (with colour photos) are given in the accompanying stitch guide.