the basic scheme is
click one of your units to select it then click a space in range to move it there or click an enemy to attack it; you can move then attack but can't attack then move
you can replace one card from your hand each turn and draw two new cards at the end of each turn, and you can play cards by spending mana equal to their cost
the amount of mana you start a turn with increases by 1 each turn up to a maximum of 9 and you can also gain extra mana temporarily by moving or placing a unit on those orb tiles in the middle