Find a doorframe with a top jamb and a sill into which you can drill holes.
Drill a small hole into the top jamb of the doorframe.
Use a plumb bob to find a point in the door sill directly below the hole in the top jamb.
Place the prism pole’s tip in the sill’s hole.
Extend the prism pole until its top fits into the top jamb’s hole, and lock the pole’s height in place.
Turn the vial’s adjustment screws until its bubble is centered.
Rotate the pole to check if the bubble remains at the center.