Swimming

Care must be taken when swimming with a monofin or the complete tail.  With it on, you cannot quickly remove your feet if there is an issue.  I prepared by watching lots of videos, and with practice. I started by first swimming without it, and just holding my legs together pretending.  This built confidence in being ok with my legs stuck together.

I continued practice with just the monofin, to get used to it's affect.  It was surprising how much easier it was to swim with it than it was just pretending as the amount of force possible with the monofin is quite surprising.  While floating in the deep end of a pool, it only took a single kick to lift myself up a foot out of the water, something I could not have done with my legs alone.  I almost wish I had switched to using the monofin earlier.

Take care with this, and have someone around just in case.

For more confidence, there are an increasing number of mermaid schools popping up around the US, so you could learn with an instructor if you'd like.

Fin types

There are a number of commercial mermaid tail products out there, mostly for children.  Most of them are a flat cut sheet of acrylic placed inside a neoprene sleeve.  The sleep simply has sock openings.  This means your feet are not firmly attached to the fin itself, and can flop around.  While I've not tried this style, it seems to me like you would not be able to get nearly as much power with it, and I wouldn't bother buying this kind.  I would only go with a kind that is strapped on, and has foot compartments molded into the plastic/rubber so that the fin moves firmly with you.  For instance any of the FINIS monofin products are like this, and are sometimes used by divers showing the actual effectiveness of them.

There are a number of monofins with foot compartments which are already shaped like mermaid flukes, but only for children.  If you are looking for something for a child, or someone who has a child sized foot, you might just go with that approach.  That said, the surface area of those are much much smaller, so would not have nearly the same power.