Help with Knitty's "penny" dog sweater

[anky]

I've started knitty's "penny" dog sweater and am up to the part where you bind off the work so it is divided into three sections to create the legholes but now I'm stuck. I don't quite understand how you can work on one section and then reattatch the yarn to the next section until all three are the same length and you join them by adding stitches. They say to use circular needles but I've been using regular needles with the idea of transferring the sections I'm not knitting onto stitch holders.
So I guess my question is how do you knit these three sections in succession as I don't know where to join the yarn.