I don't have Kirsten, so this is more or less guessing, but I hope it helps.
For getting the ends as low as possible, it might be helpful to try and divide the hair the way AG had it. That should be "slightly" easy to divide it up into the 3 sections where the tips of the hair are the same length. Since they chop it all off on the ends after the braids, the 3 sections should be (I'd guess) significantly different enough to part each side into three strands of equal end length. Did that make sense, lol?
I do have a PM mini Kirsten, and from that, it looks like the right side has the braid starting with the front strand, and the left side starting with the back strand, the braids should be pretty tight from how they look in pictures.
If multiple attempts prove hard, and you keep her as mostly a display doll now, you could always put the braids in and carefully trim off the ends so they look perfect, it would probably only be like a cm or 2 off the longer strands. I think everyone here understands being uncomfortable with that, though! Whatever you do, from your sig pic, your Kirsten's hair looks great for as old as she is.
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