Here is the pattern I used to make the wrist-warmers, with the alterations I made. (It was originally a pattern for making gloves in a 1960 Workbasket magazine.)
Sandy’s Wrist-Warmers made with Sock Yarn
Materials: Approximately 2 skeins of Sock Yarn. I used Fortissima Colori Socka Color yarn, 50 gram balls. Size 2 double-point needles. Size 2 straight needle for casting on.
Gauge: 30 stitches and 42 rows to 4 inches using a Stockinette Stitch on US Size 3 needles. (from the yarn wrapper)
With size 2 straight needle, cast on 64 stitches. Divide stitches on 3 double point needles (20 sts on 2 needles, 24 sts on a third needle), join. Place a marker at the beginning of round. Work in ribbing of k2, p2 for 3 inches. Work in stockinette stitch (knit every round). Work 8 rounds even.
Next Round (Thumb Gusset) - Work to within 3 sts of marker at end of round, place another marker, inc in next st, k1, inc in last st. Knit 2 rounds.
Next Round - Work to marker, inc in next st, knit to last st, inc in last st. Knit 2 rounds.
Repeat last 3 rounds, working in same manner having 2 sts more between increases in thumb gusset until there are 21 sts in gusset (between markers).
Next Round - Work to marker, and place the remaining (gusset) stitches on a small holder or safety pin or thread. Cast on 5 sts at the end of working needle, and join to beginning of round, skipping the thumb gusset.
Knit every row for 1 inch. Bind off. Edge will roll. If you prefer, you can do ribbing instead for an even bind off edge.
Place the 21 sts from holder on 2 needles. Pick up and knit 5 or 6 sts from body of finished mitt. Knit 1 round.
Next round - Knit even, knitting 2 sts together at the section where you picked up stitches earlier. This will help the thumb gusset lay nicely.
Knit every row for 1/2 inch. Bind off. This edge will probably not roll since it’s so small and fits nicely around the thumb.
You’re done!