Here's the pattern for the Dandy Christmas Mittens:

I used bulky yarn and cast on 40 stitches on 4 dpns.
K4 P1 throughout.

Body of the mitten: Keep going until the thumbhole (depends on how long you want your mitten to be). Then K5 with a contrasting yarn, slip those five back onto the needle, and knit again normally.

Bind off when you have reached desired length.

Thumb: pick up the stitches from the thumb hole by removing the contrasting yarn. Pick up three extra stitches either side, 15 alltogether. Knit until thumb has reached desired length, then decrease and bind off.

Flap: Pick up 15 stitches along back of mitten. I did this 5 rows in, although next time I would probably do it further down. CO (I used a cable cast on) 18 stitches. Knit in the round until flap is long enough, then start decreasing evenly and bind off.

Sow on button and make a little catch-thingy on the top of the flap.