You can also just exchange places with the first and last stitches, putting the stitches at the point of each needle on the opposite needle, kind of like passing a stitch over but you don't drop either off the needles. With this method, you would cast on what you need, no extra stitches.
Toby, I think for your original post, they are assuming that you are poised to begin knitting the first round, which would have your working yarn on the right needle and then you would slip that last stitch (closest to the point) on the right hand needle to the left and then knit what is now the 2 stitches closest to the point of your left needle together. This is how I usually join my rounds, after casting on the required number needed + 1, since you then lose that extra stitch when you k2tog. I usually also use both the working yarn and the tail yarn for the k2tog and a few stitches more to weave that tail in, giving a good tug after the k2tog to tighten everything up. This makes my joins pretty imperceptible and with the tail woven in, there's no darning of that end later. You just need to make sure on the next round to treat those double stitches as one, as there will be 2 loops around the needle for each stitch that used the working and tail yarns.