Here are the two Furled Sails podcasts where Marvin Creamer describes the techniques he used for his voyage.
He estimated the altitude of stars at meridian passage to establish latitude. This means estimating the Zenith, position, Zenith angle and when the star is on the meridian. Sounds tough! There is a trick for finding the Southern Pole from projecting the Southern Cross.
He navigated using stars as a compass and also establish wave direction from wind.
He arranged to pass through Drake’s Passage at the Summer Solstice so that he could see where he was going.