Index -> Playing the Game -> Planning Phase -> Playing Cards
Playing cards from your hand is as simple as clicking on the card you'd like to play and dragging it onto the correct target.
The mouse cursor will change to a "plus" when clicking and dragging the card. When you are hovering over a valid target, the cursor becomes a "check." When you release the mouse button over a valid target, you will hear a chime, and a purple line from the card to the target appears to indicate the card has been played. The card will also darken.
The cards you play now do not actually enter play or take effect until the execution phase.
For more information on Playing Cards, see: