When the time comes to start the armhole, you put the gusset stitches on stitch holders. I use a length of old gansey wool of a different colour – pull the wool through the gusset stitches, including the purl seam stitches on either side of the gussets, then tie the ends together and tuck what’s left out of the way inside the body.
Next you have to put one half of your stitches on a holder while you knit the other side. The simplest thing to do is to leave the other half on the circular needle you’ve been using, i.e., using the circular needle itself as a stitch holder (the stitches are already on it), and get out a new one to carry on with.
The front and back of a gansey were traditionally identical. In fact, it’s often said that there wasn’t even any shaping for the neck, so it didn’t matter which way round you wore it – there wasn’t a “front” or a “back” side as such. (But that’s not necessarily true – if you look at many of the ganseys featured in Gladys Thompson’s Patterns for Guernseys, Jerseys and Arans you’ll see that they have elegantly curved necks.)
Anyway, I don’t like a close-fitting collar or neck, so I usually shape a neckline on the front of my ganseys. As a result, the front will have an indented neckline, while the back won’t. I usually knit the back side first, and leave the shaping on the front till later.
Continue to knit whichever side you’re on back and forth on your new circular needle, continuing the pattern as you go, until you have finished the yoke – but still have the shoulder straps to do.