This is a daunting one. It's best to do this late game when you know where everything is or as you're exploring the Island.
Use charcoal to survey the area to mark it on your map.
You have to mark all named locations and icons on your map, you DO NOT need to fill the map completely. You don't have to mark the randomly generated Prepper Caches.
The best way to do this is to survey the area as soon as the location appears in the top left corner of your screen. You will also have to mark the icons on your map, such as map transitions, mines, maintenance sheds etc.
There is an in-game checklist that can help you keep track, it will be hidden by default and clicking "force reveal" will not void the achievement. If you're lost use that to keep track of what you've marked and missed.
I've found it best to stick with one region and mark off every location in that one region then move on to the next.
You also have to list all notable areas in connecter regions like Winding River etc.