It really isn’t that difficult to write a script. Once you think up what you want to say, and how you want it to be said, the rest will come to you.
First of all, you need to know a little about the stage and theater lingo. For instance, you will want to make sure your main character in a scene is down stage, and/or center stage. You will want to make sure you have your characters entering and exiting the stage in the proper areas. It would help to have a diagram of a stage to look at as you plan. You can find a good example at http://www.musical-theater-kids.com/stage-directions.html .
Second, compile your list of characters. Sometimes, this can come after you have written your rough draft. You will be surprised to see that after you get into the actual writing of the play, new characters tend to emerge. You will need to give a character sketch of each character. This will help the actor know who the character is and how to portray them on stage. Create a personality; cranky, quiet, overly protective. Create a background, such as, is this person a child, or adult, or somewhere in between. Does this character have any strange habits? What is the history of this character? Where is this character from? Think of how you would describe your self so that someone who has never met you can understand you.
Make a rough draft. While writing the play, envision the action on a stage. It helps to “see” where, when and how the play will go. I envision the people on the stage and write what I want them to do. I tend to write stage directions in italics, character names in all caps and in bold, and make sure there is plenty of spacing. Usually. I write plays for children. This method makes it much easier for children to figure out their lines.
Do a walk through. If you have the opportunity to have a group of actors read through the script, great! This will help you find any glitches you may have. For example, you have left a character stranded on stage with no escape route. This is a common rough draft problem. However, when you see the actor reading the part just standing there, you know you need to write an exit for that character.
Edit and do a walk through if possible. This way you will be able to catch anything missed on the read through and be able to rewrite before your actors start memorization. It is in this stage I tend to ask the actors themselves what they feel about this line, or that exit. I don’t always use what they say, but their insight helps develop my characters to seem more life-like.
I am not a professional playwright. I write scripts for whatever situation I need. I have written quite a few using this method and they have all turned out rather well.