Here's a link
to the pattern.
Basically, at the point where you divide the sts, youíve split the warmer into two layers, one layer in front of the other. One layer is on the straight needle and the other is on a circular needle. You work each piece separately for the given length and now youíre at the point where itís time to join them together. Hold the two needles next to each other and parallel to each other in your left hand making sure the two pieces lie flat, one on top of the other and that they arenít twisted. Use the other end of the circ or the other straight to knit the sts from the two needles, alternating which needle you take the sts from .Knit a stitch from one needle onto the right needle and then one st from the back needle.