I've been diligently working on the left mitten and arrived at the point where the thumbhole should be. The round starts on the left side of the palm so the thumbhole is created while working on the last half of the stitches on the first needle (I'm using two circulars instead of double point needles).
On the first needle I knitted the first 25 stitches according to the chart (25% of the stitches on this needle minus plus 1). I then started the top of the hole by using a loop cast on to add 22 new stitches while still following the chart. Lizbeth Upitis calls this cast on a half hitch but I found a good illustration from Wonderful Things , which refers to it as a single cast on . After casting on the new stitches, I put the bottom 22 stitches on hold using a strand of cotton yarn. The remaining stitch is knit onto the left needle. Now I can continue knitting the mitten and save the thumb for later.