Here's how I do it:
Mid-shoulder height - back my candidate up against the wall standing front square to me, feet slightly apart, arms down loose, looking straight ahead. Hold a 30cm ruler on her mid shoulder with one end of the ruler touching the wall. I use the ruler "edge up" rather than "flat up", as the ruler end will hit the wall vertically end-on and "parallel" is closest when the whole ruler end is touching the wall. I tend to use mid shoulder as my upper point, as I can easily find this spot on a pattern, but if you prefer to take centre back neck height, you can place the ruler behind her neck against the wall, and eyeball the correct height from the front.
Mark where the ruler touches the wall. I generally use half a sticky note, but you can use whatever else comes easily off the wall when you're done.
Crotch is a little harder. Place a rigid metre ruler between her legs, and ensure that the ruler is (a) parallel to the floor and (b) just touching her crotch. Again, I've found "edge up" works best if you can manage this. Touch the back end of the ruler against the wall and keep it there. I drop the front end while making sure the back end maintains contact with its spot the wall, have her carefully step over the ruler and move away, bring the ruler back up to parallel, and mark with the other half of my sticky note.
Good luck