Hi...this is what I would do....is the "easiest", but need to be planned firstly.
Are you washing her hair firstly? Make sure your choices for shampoo/conditioner and leave-in are made.
Make sure you have enough conditioner in the hair to have slip to be able to comb the hair with the least amount of struggle.
Make sure you have a comb that is big enough to handle long hair...that is to have small and wide teeth, but is deeper. Not a regular cutting comb, but one that is easy to wield.
The back perimeter length done first, then the front perimeter shape.
Comb all the hair back and make a one inch section (either side of the centre) at the top of the head from front to back. That's your guide. for interior shape/layers.
Determine the layer length at the crown by pulling a piece down to where you want the layers to start. The safest would be to use a layer that is longer than the bottom perimeter length when pulled up at the back.
Starting at the front, pull the hair up 90º, slightly longer than the front guide, cut the length and angle to the crown determined length. Continue down the guide and bring the hair straight up but slightly directed forward and cut to continue the angle. Follow the 1" section until there is no more hair to cut.
From there, take diagnonal partings starting at the back, using a portion of the guideline and holding the back up 90º but slightly directed forward as to not cut into the bottom perimeter line. Continue along the sides in the same manner but using front to back diagonal partings, lifting to the top guide.
Hope this makes sense how I explained it...not my forté....
Hope it goes well for you...keep us posted.