After your enemy-player’s turn click on "Archer" unit. In the appeared menu click on a "Move" button and on a free blue colored area. It’s better to choose the area where "Archer" unit can attack an enemy-player. If you want to see at which cells "Archer" unit can shoot click on "Attack" button in his menu. These cells are colored in yellow. A shot as any other attack will pass a turn to an enemy-player.
If you didn’t attack during this turn but want to finish a turn click on a "Turn" button.