You explore by taking the exploration ideas, which will be possible after you have unlocked your fifth idea group.

This is a bit cheeky but the only way I found how to do it pre-1444 as Native Americans.

Start as or become Nahuatl and unlock all the religious reforms to get that sweet 50 AD colonizer. Snake your way up to the arctic north of Canada, conquer and colonize up there. If you raise enough unrest in provinces with (I think) Inuit culture, Greenlandic separatists will spawn. Let them siege territory, generating Greenlandic cores. Defeat the rebels, resiege the provinces with the new Greenlandic cores, and release Greenland as a vassal.

When you are able to do so, integrate Greenland and you'll inherit maps of the North Sea and European nations. Annoyingly, you'll still have FoW in one sea tile between Greenland and Iceland, so passage between the New and Old Worlds still isn't groovy.

However, hopefully, you'll notice Iceland is independent. Maybe this works if another power is in control of Iceland, but for me Iceland was independent. You should have just enough coring range to purchase a province from them. Core that baby up, raise your new armies in Iceland, suspend your disbelief, and get going on your pre-1444 Sunset Invasion (obviously how they did it in CK2, as they usually landed in Iceland first).