1 - Set Assumed Position

Select the Horizon tab at left to view the computed horizon for given Assumed Position, shown top left.

Dials for adjusting Lat and Long pop up when you click on AP.

2 - Sight Preparation

Mouse-drag the crosshair to a celestial body to 'shoot' with your sextant, i.e. measure its altitude above the horizon.  In navigation terminology a sextant measurement is known as a sight.

You can choose from the sun, moon,  planets or 'the 58' almanac-listed navigational stars, subject of course to whether you are shooting during the day or at the twilights.

3 - Add an entry to the Sights Table

Click the shoot button (sextant icon) to enter your sextant's altitude. You finalize this new sight by clicking the dialog's stopwatch button to set its precise time.

4 - Lines of Position

Select the LOPs tab at left to view the lines of position corresponding to the Sight Table entries.

Ideally for a good fix you will aim to shoot multiple sights over a wide spread of compass directions

5 - Sight reduction detail

Select a Sight Table entry and click the detail button (...) to inspect the sight reduction housekeeping.

6 - Bibliography

A standard reference work for sight reduction and other techniques of celestial navigation is
Bowditch's American Practical Navigator
https://msi.nga.mil/Publications/APN