The pattern requires a multiple of 10 sts plus 3 extra, (eg 13, 23, 33, 43, 53 etc...). In the image above I've cast on 33 sts so that the pattern is repeated across 3 times and the 6 row pattern is repeated 3 times up.
How-To Video https://youtu.be/q8Yr789rdK8
What You'll Need
DK (Light Worsted Weight) Yarn or something finer
A Pair Of Knitting Needles
yfwd - Yarn forward
Psso - Pass slip stitch over
Slip 1 - Slip 1 st purlwise
Row 1 (RS) K1, *K1, yfwd, K3, slip 1, K2tog, psso, K3, yfwd. Repeat from* to the last 2 sts, K2
Row 2 Purl all
Row 3 K1, *K2, yfwd, K2, slip 1, K2tog, psso, K2, yfwd, K1. Repeat from* to the last 2 sts, K2
Row 4 Purl all
Row 5 K1, K2tog, *yfwd, K1, yfwd, K1, slip 1, K2tog, psso, K1, yfwd, K1, yfwd, slip 1, K2tog, psso. Repeat from* until the last repeat of the row where you will end with K2tog, K1 instead of slip 1, K2tog, psso.
Row 6 Purl all
Rows 1-6 make up the fishtail pattern. Repeat these rows to your desired length.