Draw a basic layout of the set design on a blank white sheet of paper. The drawing should be from an aerial or bird's-eye perspective. It is important to include all of the furniture and entryways in the drawing so that you can better visualize the stage when you block the play.
Read through the entire play on your own and make notes in the margins of the script where you want certain actions, such as an entrance, exit, or stage cross, to be performed. Refer to your drawing of the set design as you block the play, as it will help you remember where certain furniture and actors and such are at each moment of the play.
Gather the actors on stage and have them write down and walk through the blocking directions that you give them as they read through the entire script. It is important for each actor to write down the blocking for his character in his script, as each actor will have to memorize his blocking along with his lines.
Watch the actors run through the entire play on stage once it has been blocked. This gives you the opportunity to make sure that each actor correctly wrote down, and understood, the blocking that you gave him. Now is also the time to make sure that the blocking of the play makes sense and flows naturally, and make any changes that might be necessary.