Prism poles have been an invaluable part of surveying for ages -- the only major change in use has been what they're used for. Back when they were first introduced as a visible aid in surveying, prism poles helped surveyors to keep lines straight and angles precise. Now, they still serve that purpose but often do so with GPS surveying systems. A prism pole is only as good as it is plumb to the Earth, which makes plumbing rods invaluable parts of the surveying process.
Mount a prism pole-adjusting jig to the prism pole; generally speaking, adjusting jigs mount to the pole with clamps temporarily or with screws if designed as a permanent mount. Slot your plumb bob string into the string mount near the top of the rod; it should use either a notch or a self-centering, curved mount. Other types have provisions for a screw-in plumb bob.
Stand your prism pole on a level surface and adjust the tripod so that the pole is as vertical as you can get it. Let the plumb bob string down until the bob point hovers just over the calibration point on the lower-end of the adjusting jig. Tie the string in a slipknot for later adjustment.
Adjust the tripod legs so that the prism pole is completely plumb and rotate the pole in the tripod mount and jig as necessary. A jig can help to compensate for a slightly bent pole, so this is the time to ensure that the pole doesn't lean one way or the other relative to the jig.
Adjust the plumb bob string length as necessary and tighten the knot. Tighten the adjusting collars on your tripod. If you're not using a tripod in the field, tighten the plumb bob knot and use the jig to plumb the prism pole as you require.