Navigation and GPS

I just finished reading a mediocre book called "La carta esferica". I don't have any idea why I did not put the book and all of its 590 pages away a lot sooner, but so be it. The one thing that now I am very curious about is the use of a sextant to determine location. Considering that I still confuse latitude and longitude, I need a lot of training. SO that is now my new weird intellectual pursuit. If any of you know how to do that, please let me know and teach me how to figure it out.