Small sections of coastlines can be mapped with reasonable accuracy just by measuring the direction (from the sun/stars or with a compass) and distance (from speed and time) as one sails along the coast. Inland features can be mapped using direction (sun/stars, compass) and distance (from triangulation, or from travel time and speed).

To keep it all accurate at larger scales, measure the latitude and longitude of suitable reference points (e.g., towns/cities).

This question has come up before:

• For an overview of latitude and longitude measurement and surveying, see u/MrDowntown in https://www.reddit.com/r/AskHistorians/comments/9y1k4s/how_did_early_cartographers_produce_accurate_maps/

• For more, and how accuracy improved over time, see my comment and u/qed1 in https://www.reddit.com/r/AskHistorians/comments/bao24n/how_were_maps_so_accurate/

• A nice example of accurate modern mapping in the 19th century is the Great Trigonometrical Survey of India (which led to the first measurements of the heights of Mount Everest, named after George Everest, who was the second superintendent of the Survey) and other Himalayan peaks. See https://commons.wikimedia.org/wiki/File:1870_Index_Chart_to_GTS_India-1.jpg for a map of the survey points. For a history of the Survey of India, see Strahan, C. (1903), "The Survey of India", Professional papers of the Corps of Royal Engineers 28, 141-171: https://archive.org/stream/professionalpape28grea#page/n211/mode/2up (this was discussed in https://www.reddit.com/r/AskHistorians/comments/aauezl/before_the_invention_of_modern_equipments_such_as/ which discussed measurement of mountain heights rather than mapping in general)

Thank you