From the book:

"Even first-time knitters can whip up these adorable bunnies in a flash.  The entire slipper is knit in garter stitch (knit every row) on straight needles.  Designed to fit a toddler, the slippers can be made larger or smaller by knitting more or fewer rows, respectively, before decreasing for the toe.  You can also adjust the finished size by felting (agitating) more or less."