Map and Quill

To start mapping an area, you first need to purchase the map from Cornifer, which will allow you to see the uncompleted map of the area. In case you miss him in an area, Iselda will sell the map of that specific location for a slightly higher price in Dirtmouth.

To complete the map, you then need the Quill, sold by Iselda. Once you explored uncharted areas, you will need to rest at a Bench to update the map.

The Wayward Compass charm show your location on the map.

Different pins can also be acquired that shows different points of interest on the map.

Maps
Maps can be bought from Cornifer in each area, or, if the player misses him, Iselda sells them in their shop in Dirtmouth.  

Map Pins
Map pins are used to mark locations on the map. They are sold by Iselda in Dirtmouth, with the exception of the Collector's Pin that shows Grub locations on the map, dropped by The Collector.