A theodolite is a surveying instrument that is used to measure horizontal and vertical angles. It consists of a rotating telescope mounted on a tripod, which can be used to measure angles in both the vertical and horizontal planes. The following is a step-by-step guide on how to use a theodolite:

1. Set up the theodolite on a level surface, making sure that the tripod legs are evenly spaced and the instrument is stable.
2. Adjust the instrument to level using the foot screws and bubble levels on the tripod and theodolite.
3. Extend the telescope and focus it using the focusing knob.
4. Turn on the theodolite and set the display to show the desired units (degrees, mils, or grads).
5. Sight the target using the crosshairs in the telescope. Make sure the crosshairs are aligned with the center of the target.
6. Use the horizontal and vertical adjustment knobs to center the crosshairs on the target.
7. Read the horizontal and vertical angles from the display. The horizontal angle is the angle between the direction of the telescope and the reference line, and the vertical angle is the angle between the direction of the telescope and the horizontal plane.
8. Repeat the above steps for additional readings, taking care to maintain the same reference line and level for each measurement.
9. When finished, carefully lower the telescope and turn off the theodolite. Remove the theodolite from the tripod and store it in a safe place.