Honestly, Indigo, I drew a picture of a betta, then broke it down. Body shape, then finnage.
I do the main body in one piece UNLESS the head is a different color than the body, then it's fleece or felt first, then I cut two slightly larger pieces of the body color, and sew them over the head color as I go. It's much quicker, but attaching the dorsal to the back without a seam (like Nereus) is a pain.
I use embroidery thread, and a large needle, it makes it easier on my hands, but there's times I have to break out my pliers to pull the needle through.
I stuff with fiberfill.
That's all I can tell you, the rest is trade secret. It's makes a Skyewillow betta different than the 2 dozen on Etsy. Syriiven is going to take a whack at it too, and I get to see 6 Skyewillow bettas daily till these fellas ship, so I'm looking forward to seeing a Syriiven Betta, and an Indigo Betta's Betta!