Step 1: Correct for Index Error as described elsewhere on this site.

Correcting for Index Error

There are several ways to go with this, depending on who you ask. The first method involves a little bit more prep-work, but can enable you to find the specific body you're looking for a little easier. Start by calculating the altitude and azimuth of the body and preset the sextant to that altitude and point it in the right direction. From there you simply tweak as needed until the body can be seen on the horizon.
The second method is as follows:
Step 1: Set everything to 0.
Step 2: Point the sextant at the body you wish to shoot.
Step 3: Squeeze the spring clips on the index arm together to release it from the track and slowly slide the arm down the arc, keeping the body in sight.
Step 4: Drop the body until it rests on the horizon.
Step 5: With the body resting near the horizon, adjust the micrometer drum until the body rests exactly on the horizon. For the sun or moon, the lower limb should be on the horizon; stars and planets are enough of a pinpoint to set directly onto the horizon without worrying about upper/lower limb.
Step 6: Tilt your upper torso in an arc, holding the sextant still in your hands. This is to ensure that the altitude you measure is exact and that you are not standing or holding the sextant slightly askew.
Step 7: If, while tilting, the body, at it's lowest point, falls above or below the horizon, readjust the micrometer drum until it sits perfectly on the horizon.
Step 8: Note the time exactly and note the measurements on both the arc and the vernier.
Step 9: Reduce via a sight reduction sheet and plot your position.