Thanks for trying out the game, we didn't get round to making nice clear UI for the controls, but it works like this:
WASD => movement
You can cast a spell from any appropriate tile that is contiguous with a tile that you are adjacent to (up, down, left, right). The appropriate start tile(s) are shown under the spell name, and will move each "turn".
e.g. If you are standing next to a body of water, you can cast the water wave spell starting from anywhere in that body of water (make sure to drag to pick the direction)