TL;DR: To navigate using an astrolabe and a watch, measure the sun’s altitude at local noon with the astrolabe. Adjust for the sun’s declination from an almanac. Your latitude is roughly the corrected sun altitude. For longitude, compare local noon to the reference time on your watch; every hour difference equals 15 degrees of longitude. This method involves basic trigonometry for calculating angles and positions but is quite approximate and not highly accurate for longitude.
This allows to navigate beyond the horizon from the coast so you can travel overseas and beyond where other people has been on a ship without following the coast.