Answer:
Left and Right:
The parts of a stage are labeled from the actors' perspective.
(Try to imagine being on the stage instead of looking on from the audience.)
Therefore, when you label parts of the stage, your left is the actor's right, and vice versa.
Up and Down:
Downstage and upstage exist because a long time ago, stages were tilted upwards so people in the very back could see the action.
So, the farther away you are from the audience, the further upstage you are. If you are close to the audience, you are downstage.