There are tons of points of interest to explore on the map of Ghost of Tsushima. Lighthouses are really not the most interesting places, nor the most difficult to find, since they are always on top of a cliff on the seaside. As long as you walk along the coast, you should find them easily. Freeing places occupied by Mongols will also help you to find them.
However, since the Light the Way trophy requires you rekindle them all, and knowing their location will save you time if you don't intend to search the whole map. Some of the lighthouses are linked to a mission or a progression in the story, the very last one will have to wait until you're almost done with the story in Act 3.
Lighting a lighthouse will advance your legend. Note that there are often artifacts nearby, such as records or banners. You should equip the Traveller's Attire to kill two birds with one stone.
Act I Lighthouses Location
Act II Lighthouses Location
Act III Lighthouses Location
You must participate in the final battle in order to light the lighthouse in the Northeast.
Original content by Robin "Raiden" Bouquet.