It was pretty easy for me to adapt to swimming in a tail, but then again I had been swimming competitively for 13 years before I got into a tail.
My best advice would be to practice swimming without the tail and then add it once you've got your technique down. When I first started swimming, we had to start with the basics before we could build on top of it. This included getting to swim with fins.
A good way to practice your dolphin kick, if you're first starting out, is to kick on your back in a streamline. This forces you to focus on kicking from your abs and not your knees. (plus you get to breathe the entire time!) When I'm coaching, I always make my beginner swimmers kick on their backs when they're first learning how to dolphin kick just because it's so effective!
Your dolphin kick starts from your abs and flows all the way down to your toes. Your abs should be throwing your legs down and your hamstrings should be pulling your legs back up. Lots of people know there's a downward kick but many people forget about the up kick
(which hurts my little swimmer soul). Your knees shouldn't really bend. They're supposed to carry the flow to your toes and that's basically it. Your toes should be pointed the entire time. The more you can point your toes, the better.
I see so many mermaids with bad technique
(which pains me to watch even more). So much could be fixed if they took some time out of their tail to actually practice their kick! People get so caught up in the magic of the tail and monofin that they think they can practice without it. Always start with the basics before adding anything fancy to your swimming (or in this case kicking). Once you've got your technique down and can consistently dolphin kick at least 250 meters nonstop (on your back or stomach) without feeling super tired, then I'd say you're ready to add your tail and monofin.
I have a couple videos I recorded of me swimming for animation references on my youtube channel. You can check those out if you want a visual of good dolphin kick technique.
Reference 1
Reference 2
And I also have a video of myself doing tricks in my monofin on my mermaid channel, if you want to see that too.
Monofin Video
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