I'm not an expert at doing this yet, but I've been told that to knit something like this in the round, you just double the number of cast on stitches, and place markers at the 1/2 way mark and at the beginning of your "round" - that way you can make any decreases near the markers as though there were seam lines.
So in this case, to make the smallest size, the directions say
Cast on 64sts
So, to knit in the round I would CO 128 sts, with a marker between sts #64 and #65 and another marker between sts #1 and #128.
If anyone else knows more about this, I would LOVE some help here.[/b]