Turn your pants that you are using as the pattern wrong side out. Line up all the seams and fold in like pictured so that the center is on the outside. Lay on top of your fabric that is folded in half. With a fabric marker trace along the top around the curve, down the leg and along the bottom edge (make sure to give yourself a generous seam allowance). Flip over and repeat .
Now you should have two pant pieces. With right sides together sew along the curve.
Open the fabric up and bring together the bottom of the curves on both pieces. This is the crotch. Pin here and sew along the inseam of the pants (the long edges extending from this point).
You should now have something resembling pants.
Fold in half and slice down the side (you’re opening up the side of the leg to put the fin in). If your pants are big compared to your pattern trim off any excess.
Now you should have pants with slits down the sides. This next step is not pictured. Cut a strip 5 inches by however long you need the waist band. You can get that number by measuring across the top of your pant piece and multiplying by 2.
Hem the bottom edge (yes you still want the slit to be open). I did this by serging the edge and folding under and sewing.
Next draw yourself a fin. I just free handed it and held it next to the pants and adjusted until I thought it looked right. With your fabric folded cut out 2 times (so you have 4 pieces, 2 in each direction) and then cut two pieces out of the interfacing.
Take two fin pieces right sides together and sew around the curve. Repeat for the other fin and snip to the sewing line to help it lay flat when you flip it.
Turn right sides out and insert interfacing. Trim off any that hangs over the edge.
Tuck the fabric in along the bottom (if you need to you can trim off some more of the interfacing.
Sew along the bottom edge.
Place fin with the straight edge alined to the bottom edge of the pant.
Sandwich in between the front and back part of the pant leg and sew down the side. Repeat on the other leg.
Sew the shorts sides of the band right sides together so it forms a tube. Now fold the fabric so the rights sides are showing. Take the band and pin it right sides together to the top of the pants. You want the raw edges positioned up and the fold pointing down.
Sew around leaving a gap to insert elastic.
Measure your elastic (a few inches less than your kiddos tummy) and string into the casing and fasten. Bend the band back and close up the hole
Admire your adorable mermaid pants!