first, you need a monofin-or else swimming would be pretty hard-i bought mine off of swim2000.com, it was about $45.
Get a large piece of paper, and trace your body from your hips to your ankles.
trace the monofin on the same piece of paper according to where your ankles and feet are.
cut out the shape of your body and the monofin
fold the fabric so that the sparkly side is on the inside, then put your cutout on top of the folded fabric and trace your body shape onto the fabric using a piece of chalk (so you can see it and it wont show when you are swimming.)since i already made my tail awhile ago i used a leftover piece as an example.
put pins on the line where the chalk is to keep the tail in place. Then start sewing on the chalk line-take out the pins as you sew (if you're hand sewing like i did) i took out 4 at a time when i made mine. (sorry i dont have a picture!) make sure that you leave the whole end open-aka where the monofin goes so you can get it in and out of your tail!
i dont have a sewing machine so when i went to my grandma's house we went over my stitches with her sewing machine just to make sure that they'd stay.
i sewed on ribbons on the end so i could close the end and to make a pretty and flowy fluke. :) and.....YOURE DONE! now you can be a mermaid! trust me its soo fun :D