Use the script to help guide the actor's movements. Entrances and exits are clearly marked in most scripts.
Keep your audience in mind. Think of the stage as a framed picture. Carefully assess how you want the painting within that frame to look at any given moment. Remember to also consider that your actors need to be heard. Make sure that they are not facing the back wall when speaking.
Learn the terms used in stage blocking. Center stage, stage right, stage left, up stage and down stage. Right and left refer to the actor's right and left. Up stage is towards the back wall of the stage and down stage is towards the audience.
Let the actors have some input into their movements. They may have a natural feel for when they need to move closer or further from another actor.