While viewing one of your regional maps, hover over a tile on the map and you should see a gray "+" icon appear. Click this icon to open the tile form modal window. The form gives you many options to edit your map tiles.
Add a Label to your tile that will appear on the map. Use this to mark points of interest.
If you are a GM, you can use the "Secret?" toggle to hide the details of the tile from your players. This allows you to pre-populate the map with content and turn it on as your players explore.
The blue "Quick Create" button works the same as the button on the bottom of the form. It is just a faster way to create or update a tile since it is in a more convenient location.
The Status dropdown affects how the tile appears on the map. Mark the tile "explored" to hide the overlay and see the map image underneath that tile. Mark "unexplored" to keep the overlay on the image. Mark "discovered" to keep the overlay, but make it partially transparent so you can slightly see the image underneath.
The Color dropdown allows you to color the overlay and icons on that tile. Use this to denote various factions on a map, owned lands, danger areas, etc.
You can link a map tile to a Place by selecting one in the next dropdown. This will add a link in the tile details window.
Keep additional notes regarding tiles using the Notes text editor. These are visible once you click the tile.
You can also select up to three icons to display inside a tile. These are helpful to mark things like cities, forts, campgrounds, treasure, danger, etc.
When you are done, click the blue "Create Tile" button at the bottom.