Using a sextant is a skill, like firing a pistol at a moving target, or patting your head and rubbing your belly at the same time. It is particularly difficult on a small boat in heavy weather.

The procedure is to get the heavenly body and the horizon simultaneously in the field of view of the instrument (it's done with a split-surface mirror superimposing both images).

It is also necessary to make sure the sextant is oriented perfectly vertical at the instant of observation, which is accomplished by rocking it gently so that the image of the star describes an arc above the horizon (what you see through the telescope) and gradually manipulating the micrometer knob until that arc just grazes the horizon. At that instant you call out so your assistant can record the exact time, and then you read the elevation angle off the micrometer at your leisure. If the instrument is properly calibrated and adjusted, that is the angular altitude of the star at the instant of observation. Then comes applying corrections and solving the spherical triangles that give you location of the line of position. Fortuantely, there are reduction tables available today which allow this calculation to be made by anyone who can add and subtract angular measure.

You need a nautical almanac with astronomical ephemerides, sight reduction tables to do the calculation (I've programmed my HP48 calculator to do this) and the exact Universal Time
to correct for the rotation of the Earth.