Every playwright has a process unique to them and this process will begin to shape how you develop ideas. The process I am going to outline for you is a helpful guide to keep you moving towards completion. Tell your story, and along the way you will find that even though you will have plotted out where you wish to go with the story, it can change and you will find that as the author you discover the play more than create it.
The process of writing a play is hard work and requires a lot of discipline. Sometimes you will feel inspired and the words flow easily from your head to the page and sometimes it will feel like an insurmountable task to complete a simple exchange of dialogue. The audience will never know the difference. Keep writing. If you find that your character has run into a problem that you cannot easily solve, good! Both you and the characters are in the exact same situation and if you do not know how to solve the problem neither will the audience. In those instances, think and troubleshoot. When you have an idea use it and see where it goes. If you find that two scenes are similar then don't be afraid to get rid of one. If you find that along the way the plot needs to shift, then shift it in the outline. Trust yourself and remember that you can always go back.
When you begin writing you will need to start with a concept or idea, if you do not have one then go to the idea store and shop around with the various exercises. Once you have an idea, begin to research materials related to it and expand your knowledge.
Once you feel you are familiar with the subject matter ask yourself what the central message of the play should be. This will guide you, but it can always change along the way. You want to have a reason to write the play and to keep going on hard days. Once you have developed a theme you are ready to begin creating a central conflict. First ask yourself what kind of story you want to tell and what the conflict should be. If the conflict is external and it is a buddy love story, then you can place it in a specific location that will best tell your story.
Once you know what subject you are writing about, have a theme, conflict, story type and setting, you will find that the rest of the ideas start to fall into place. Your next step is to create characters based around the conflict. Once the characters are created and narrowed down to who you would like to be in your play, assign each character you have chosen an archetype to serve as in the story.
When you have assigned characters to archetypes now you know whose story you are telling. You hero needs to grow the most throughout the play. Don't give your best lines to secondary characters, give them to your lead. It is your hero’s story not anyone else's. Your hero serves the plot and theme and your hero is who the audience grows beside. Now that you have the Theme, Conflict, Setting, and Characters decide what your Inciting Incident will be and what you want the climax of the play to be. Once these elements have been selected, create an outline where you craft the plot and structure the events into a logical build that capitalizes on raising tension and connecting you from Inciting Incident to Climax.
Break the plot into scenes and begin writing. Do NOT edit as you write, rather finish the scene and edit afterwards. Complete each scene and make sure that each character is serving the plot, is pursuing their objective, and if a character is failing to do so revise their part or cut them from the show.
When you have completed your play, read it out loud to yourself. This will help you find mistakes that you would not otherwise see while reading it silently to yourself. Your next step is to invite actors to come read your play out loud and to hear how the play flows and see if the story works the way you want it to. See if there are any unnecessary scenes and cut them, see if there are missing elements and add them. Then have another reading.
When you finally feel like it is ready to be staged or if there is interest, give it to a company and see what they do with it. This is the part of the process where you give away control and you can see how your play will be perceived by your audience.
The hardest part of theater is always feeling like your work is not good enough and then having it put on display for the audience to judge. Don’t forget that your audience wants to enjoy your play. They got took the night off, got babysitters, and paid to see your story. They walk in giving you their full attention and it is yours to lose. Through this process you will have the highest highs and the experience the lowest lows, but the audience will tell you the truth. Listen to them. Make sure that your work is performed for your target audience and listen. If something is missing, fix it or leave it alone and move on to your next project.
Inevitably there has to be a time where you complete the project and move on. You have more ideas and more stories to tell. Congratulate yourself on accomplishing something extraordinary and start a new project.