Help with Knitty's "penny" dog sweater

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.

Once you finish that first row where you bind of stitches in two places for the legholes and turn your work to start working the first wrong side row, you will work the first section of stitches and then obviously stop where you had bound off stitches. You can then take a new ball of yarn and start knitting the second section with the new ball. If you are unsure of how to reattach yarn, you can either just start knitting with the loose end of the new ball and weave the end in later or you can weave the end in first, really close to where your first stitch will take place and go from there. So you will complete the second section with that ball of yarn and then do the same thing for the third section, adding another ball of yarn.

So you won’t have to keep removing the stitches and putting them on stitch holders and back onto the needles all the time. You can have all three sections going with three seperate balls of yarn on the SAME needle, that way you are sure to have the same # of rows per section and thus the same length per section by the time you finish your 1.5". You can do this on either straight needles or circular needles, it doesn’t really matter.

Then the pattern has you cast on stitches again which closes up the leg holes.