bc5d6644_sle_Knitting___digital_design___Mitten_instructions
[Comments] Removing the waste yarn gives a hole for the thumb without cutting the work. A thumb that seems a little big at the base seems to wear better than a tighter one. You can decrease the diameter as you knit the thumb. I also like to knit any stitch near the ends of the hole that seen to be streched a little twice – that is, knit the stitch, slip it back and knit it again. The thumb wears better with a little more ease in it. [Mitten instructions] 
Using 2 or more needles, pick up the stitches above and below waste yarn, then remove the waste yarn. Pick up 1 or 2 stitches at ends of opening.
YOFIK | Your Own Fair Isle Knitting – digital design!
Mitten instructions