For Madeline's Mermaid Tail I used Simplicity pattern 4043.
Incidentally, a word about buying patterns - never (ever!) buy them at full price! Every few months Joann's has McCall's and Simplicity patterns for as little as 99 cents. Keep an eye on their ads!
Not much to say about this part. I followed the instructions pretty close, except for the sequin aspect. They had suggested buying sequin trim (pre-sewn sequins that you just have to baste in place), but they are very expensive. I thought I could do a good job by hand so even though I had never done it before, I gave it my best shot.
Again, the method is really easy - sequin, bead, and then back through sequin hole again. I used seed beads and these plastic pearl beads I got from my Mom the last time I visited her house. (I pilfer craft supplies, what can I say?)
I sewed them on both sides of the tail. I alternated between the sides on every sequin/bead to make it easier on myself. In other words, I'd do one sequin/bead and then I'd flip it over and do a sequin/bead in the same spot on the other side.
I think it turned out well. It weighs a ton - not literally, but for a four year old, it was a tough proposition to carry it around while trick-or-treating.