Build a early military unit such as the Archer requiring "Archery" and later when you get the "Construction" tech pay to upgrade it to a composite Bowman.
The unit must be inside your civilization border while you have the gold required to perform the action. if a unit can be upgraded you will have a action looking like the achievement picture on it, while doing it's turn. it's there but disabled if you don't have the gold or are outside your civilizations borders.