bc5d6644_sle_Knitting___digital_design___Mitten_instructions
[Comments] You can put the thumb where you like – it can be centered on the side of the mitten for reversible ones, or on the palm for left and right mittens. Be sure you pay attention to the second mitten! It is surprisingly easy to knit 2 lefts or 2 rights. [Mitten instructions] 
Decide on thumb position. Knit about 1” of stitches where you want the thumb to be onto a piece of waste yarn, then slip these stitches back to the left needle and knit them as usual.
YOFIK | Your Own Fair Isle Knitting – digital design!
Mitten instructions