There are loads of different spells that work for becoming a mermaid, here is the one I know best:

1) Get a Pisces flower. This is a special kind of flower, with seven almond shaped petals that are sea-green. You should be able to find it under water, about a quarter of a mile from shore. When you have it, smash it into pulp and mix that pulp with fresh sea water. Leave out in the full moon's shine overnight.

2) Eat that mixture, and close your eyes. DO NOT OPEN THEM.

3) Walk to the sea, still with your eyes closed. On the way, repeat this mermaid chant: Iv A Von, Sartis, Menamon, Sartis. Iv A Von, Menamon, Jagsta, Kestar, Sartis. Iv A Von, Sartis, Menamon, Sartis. Iv A Von, Venamon, Casta, Mex.

4) When you feel your get touching the sea water, keep on going until you are completely submerged. Open your eyes. Whatever you do, don't resurface until your sight darkens completely. Your legs will feel numb and you will feel tired, but don't worry, this is the process of becoming a mermaid. When you wake up, well. You'll see. ;)