I used nail polish remover (I hope my sister doesn't mind) to take away the flower on her right leg (this was the Spring Fluttershy) and then simply cut her hair off with scissors. I saved the hair in case I ever get into rehairing ponies. I then took the clay mane and tail and simply attached them to her. I used carving tools for the lines in her hair to make it more cartoon accurate. Once that was done, I put her on a pie sheet and baked her in a normal kitchen oven at 275 degrees Fahrenheit for 15 minutes. If you follow those instructions, your MLP toy can survive the oven, just don't go hotter or longer. When she came out, she was kinda flimsy and felt like jello so I cooled her with my desk fan. Her tail kinda started coming off of her butt after she was done baking, so I used some super glue on it, but I still worry it may fall off, this figure is very fragile now.
So that's it! This sure was a fun project and I'm very pleased with how she turned out! Now I must get Rainbow Dash and Rarity and sculpt some hair for them! Although I am kinda iffy about doing it to Rarity, her hair is already fitting enough, but Rainbow Dash needs a custom for sure! And then I'd better go get the CMC and do the same!
Let me know what you guys think in the comments and give me some advice for future customization if you have any.
Special thanks to ~Amandkyo-Su for giving me tips and advice on how to do this.