If plays are the small siblings of movies, then a one act play is the infant of the family. One act plays are short plays, but complete stories. They are plays that take place in only one scene and are generally less than an hour long.
Choose a subject to cover in a brief one scene act play. A short story works best. Remember to give the one act play the necessary plot, action and characters to make it a complete story. Research other one act plays to get ideas and inspiration for yours.
Develop the action first, then compose the dialog before you decide anything else. Keep the plot simple for a one act play and it should move consistently throughout the play.
Develop the characters. Write out a character sketch beforehand to help you flesh out your characters and bring them to life. Give your characters a motive in life (or lack thereof) and up the stakes by making them face a problem. This is central to any story.
Generate the setting. The setting for a one act play will be one scene, but you have to still develop the scene so the audience sees everything about the story line. Include as many of the five sense as you can. Lighting helps the setting. Make sure you write in notes about how the lighting should look.
Add in the stage directions after you write the action. Write notes about how each character should respond and what props you'll need. For example, if the characters should be facing another direction and talking to another character, note it in the script.
Find performers that fit each part. Hold auditions to find the right actors and actresses. Be upfront with them about whether this gig is a paying gig or a volunteer gig.
Make copies of the play for each cast member. Save the document in case you need extra copies. Give copies of the one act play to each member of the stage and prop handling too.
Practice the play. Ask for feedback from all the people involved in the play. Hire or ask an expert to help with the production too. Hold one final practice before opening the show. This should be a dress rehearsal. Treat this final as the real thing and tie up any loose ends.