10 easy steps to become a mermaid?

There is no known way, easy or difficult, to become a mermaid. There is no scientific evidence to suggest they exist or don't exist, but that is not what is being asked.

As just a novelty, here is an incantation if you want one:

"I want a tail, no one should know. Pinch me and not make my mermaid go home. Prince, love, and all of the above, I make my commitment to all my Mermaid Prone."

But these are likely just Words and will likely not do anything.

People do have a right to believe whatever they want and given answers that are consistent with their belief systems. There is no harm in just giving someone an incantation if they just want one without trying to debunk or moralize and letting them try it for themselves. Actively deciding for another what is true or not is not science.