Jlun2 over in the Discord needed assistance with drawing Sirens so here's a not great tutorial that generally sums up how you want to go about it. Tagged Rainbow Rocks 10th Anni as it's made to help in the event though I understand if the mods are like "nah, sorry". Also yeah I know the event is almost over lol, but since there's no Siren Tutorial on the site yet, this is gonna work until someone smarter than me does one better.

So with Sirens you're going to approach posing them as if they're elongated ponies with cut off legs. A normal Pony is generally broken into 3 major circles: the head, the front of the body, and the back half of the body. For a Siren, you remove the third circle, as well as the legs, and instead create an elongated "tail".

How someone goes about it is generally up to the artist but the thought process is going to be that you replace the hind quarters with just one large tail. So you'll see with the represented pony at the bottom (Yukon) that I've given him a fish tail. Just take something like that and grow it to match the size of the siren's body.

As for the arms, as you can see >>1622286 you want to stretch those out and make them more thin. If you ever drew Celestia or given your alicorn an exaggerated body like Fleur, then it's the same idea. Stretch the snout, stretch the arms, and it works.

Finally for posing, if we were to copy, say, >>3441652 , you draw the front half as you normally would but then modify the tail to try and match the flavor of the pose. So for that pose in particular, you can curve the tail like normal, or flip the tail and have it rest across the ground or even do something more dramatic.

Be aware that usually, sirens are several times larger than ponies, unless you do something similar to the tag kellin quinn, where artist LightningBolt has him purposefully smaller as a sexual dimorphism kind of ordeal. However, this generally shouldn't change the thought process. Don't forget to use redlines and blocking in techniques when you create your Sirens!