Waistcoat stitch in a SC done in the post between two stitches. It looks like a V.
Stitch count in bold has multiple rows to it.

Ch = Chain
SC = Single Crochet
Inc = 2SC in the same stitch
Dec = 2SC together
WC = Waistcoat stitch
St = Stitch
Sl St= Slip Stitch
Sk= Skip x stitches

Start the wrist warmers.
Round 1: Ch 25 Join with a SC into the first chain.
Round 2: SC around. (25)
Round 3 -15: SC in waistcoat stitch around. (25)

Start thumb shaping

Round 16: SC 24, Inc in last stitch. (26)
Round 17: SC around. (26)
Round 18: SC 25, Inc in the normal SC place, between the two WC Sts, SC in last stitch. (27)
Round 19: SC around. (27)
Round 20: SC 25, Inc in the next St, SC in the last St. (28)
Round 21: SC around. (28)
Round 22: SC 26, Inc in the normal SC place, between the two WC Sts, SC 2. (29)
Round 23: SC around. (29)
Round 24: SC 26, Inc in the next St, SC 2. (30)
Round 25 -27: SC around. (30)

Finish thumb shaping

Round 28: SC 25, Ch 1, Sk 5. (26) (the end of this row is joining to the beginning of the next row)
Round 29 – 30: SC around. (26)
Round 31: SC 24, Dec. (25)
Round 32 – 36: SC around. (25)
Fasten off and finish.

Video tutorial from my YouTube channel below.