Turn the garment inside-out. With a 3.25 mm (US C/3) crochet hook and Colour A held double, beginning at the ribbed hem, crochet a chain along the knit column on the side of the garment as follows:

Step 1

Insert hook into both legs of the first knit stitch next to the ribbed hem as if to pick up and knit, pull up a loop, leaving a 10 cm/4" tail.

Step 2

With one loop on hook, skip 1 stitch along the column of knit stitches and insert hook into the stitch after, pull up a loop and pull it through the loop on the hook.

Repeat Step 2 along the knit column. When going over the Colour B stripe, simply chain 2 (wrap yarn around hook and pull through, twice). Continue working in this manner to the underarm. Cut yarn, pull the tail through the last loop on hook, secure and weave in the ends.

You can find Valerie's videotutorial here.