You will probably BO a few sts to make the leg openings so what you do is knit x sts, drop that yarn, BO y sts with new yarn and knit X sts, drop that yarn, BO y sts with new yarn and knit x sts. It should look something like this -
Then on the next row, work the sts to where you bound off, drop the yarn, pick up the other yarn and work the 2nd section to the next gap, drop that yarn and pick up the last one to finish the row. Do this for 2" and finish up with a WS row. The next row on the RS you will Cast on sts to join the separate sections back into one piece again.
sue- knitting heretic