It's a simple graduation of layering with a weightline on top.
Ex: imagine looking at a client face to face. This client has medium to curly hair. When the cut is completed, it will have a triangular look or bell shape to it. The bottom will have layering and the top essentially one length.
Start at the hairline or flat portion of the head and cut at zero elevation. Once you reach the ridge, place your comb on the part. That determines your elevation. As you move into the round portion of the head, you determine the amount of elevation. Also, one thing to consider: As you continue moving up the head, you can determine whether or not to pull away from the head to get more of "The Stack".
Happy New Year