How do you get the mane shorter but keep it thick for W/P?

no, cut left and right.
If you really hate the look though, after you cut left and right, you can cut up and down. The problem with up and down is it always turns out uneven

That's what I was going for...if she doesn't want a straight across mane that looks like it was just cut with scissors you cut vertically and it gives the hairs a more "rough" edge to them and not completely straight. What is also nice about that is it then doesn't have to be perfect. If you only cut straight across and a few hairs here and there aren't exactly right it will look bad, whereas with the up cutting version the hair lengths vary a little bit all over but it still looks really nice.

no I meant un even as in half the mane is short while the other half is long.