Oh my gosh, you all like it so much xD I never expected that much of a positive response!
It was actually pretty easy. For those who want to try it:
-First, I used the conditioner + hair straightener + pet slicker brush on her to remove the floofy mess of curls. To straighten the mane easily, her head has to be removed.
-To prepare for cutting, I separated her mane into two halves: Green/yellow and yellow/orange/red. I took a bit of the yellow from the two inner rows of plugs and brought it forward as well, this way she didn't get left with an ugly part in her mane. I kept the upper and lower sections separated with hair clips.
-then, I removed the front clip, held her head-down over a sink, and poured boiling water over her forelock, making sure the hair remained straight. I draped it over her forehead and let it dry that way.
-I gave her a layered cut, with nothing more than household scissors. An easy way to get a layered look started is to hold a vertical section of hair out from the natural curve of the pony body, and cut it to the same curvature. Always cut it a little longer than you want the end result to be, as there will be more trimming involved afterward.
-I also trimmed and layered her tail hair.
Thanks so much for all the comments! One day I'll probably add some more colours to her tail and perhaps the mane too. It'd be nice to have a more show-accurate version, but really, any more than 4 colours in a mane or maybe 3 in a tail is just going to look messy (unless I use styling product on her, but I want to be able to mess it up and play with her, lol).