To decrease mitten on circulars:
Row 1: k1, k2tog, k6, k2tog, k1 repeat to end of round.
Row 2: k.
Row 3: k1, k2tog, k4, k2tog, k1 repeat to end of round.
Row 4: k.
Row 5: k1, k2tog, k2, k2tog, k1 repeat to end of round.
Row 6: k.
Row 7: k1, k2tog, k2tog, k1 repeat to end of round.
Row 8: k.
Row 9: k2tog all around. Pull thread through to tighten.

To decrease thumb on circulars:
Row 1: k2tog, k5, k2tog, k5.
Row 2: k2tog all around.
Row 3: k all around. Pull thread through to tighten.