Add a costume to your child's doll dress-up with a mermaid tail. The mermaid tail is customizable for a small doll to a large doll. Any yarn can be used. Try a sparkly ribbon yarn to get the look of scales, a natural yarn for softness or an acrylic yarn for ease. Check your gauge on the yarn used and the measurements of the doll from the waist to the feet to ensure that the tail is big enough.
Multiply the number of stitches per inch by the waist measurement of the doll to get the cast-on number. Cast on the desired number of stitches onto one needle. Divide evenly over three needles and join to begin working in the round.
Knit one stitch then purl one stitch. Repeat to the end of the round. Continue in knit one, purl one rib for half an inch.
Work in stockinette stitch until the piece measures the desired length to the doll's feet. Stockinette stitch is worked in the round by knitting every stitch on every round.
Divide for the mermaid fins. Thread the waste yarn onto the yarn needle and slip half the stitches to the waste yarn to work later. Divide the remaining stitches evenly over three needles.
Knit the first round, placing a marker at the beginning of the round and at the halfway point. Knit two stitches together, knit to the next marker, knit two stitches together, slip the marker, knit two stitches together, knit to two stitches before the end of the round, then knit those two stitches together. Knit three rounds without shaping. Repeat the last four rounds until you have less than five stitches remaining.
Cut the yarn, leaving an eight inch tail. Thread the yarn onto the yarn needle and pull through the remaining stitches. Pull tight and weave in the end on the wrong side of the work.
Repeat steps 4 to 6 for the other fin.
Tie ribbons or glue shells to the tail for embellishment.