There is single Flag in 1 of 3 colors across 6 Landscape tiles (there are 2 tiles of each color Flag in the game):
A green Flag is always associated with Forest, a yellow Flag with Grain, and a red Flag with Village tiles.
Follow the usual placement rules when adding a Flag to your play area just like any other tile.
Note, however: At the end of the game, you only get points for Flags in territories that have been closed-off, i.e., in territories that do not have an open edge.