Have them pull up their pants until the crotch is against their body.
Starting point: at the front of the torso, place the tape measure at the dip between the collarbones. Ask them to hold it there.
Measure down and under the crotch, have them bend their knees to allow it to pass through by their thighs and rest against the crotch.
They can stand upright again.
Bring the tape measure up to the back of the neck.
DO NOT press the tape measure against the curves in their back.
You can ask them how comfortable it feels, and they may move the tape measure at the crotch until it is adjusted to be comfortable.
Ending point: the back of the collar. Imagine they're wearing a shirt with a collar. The seam of the collar will fall across a vertebrae at the neck. If they were to look down at the ground, this vertebrae can sometimes become more predominant, but be sure they are looking straight ahead when you take this measurement.
Pro-tip: if they jump in cold weather and wear bulky sweaters/hoodies under their jumpsuit, they will need to wear the bulky clothing while being measured. Especially for this measurement.