Scrite is a free and open-source app for Windows and Mac Desktops (soon on Linux also) using which you can create screenplays. This video is a demo of the Scrite app. The voice over in this video is in English, we will have another video in Kannada as well. PS: Kindly pardon all the bloopers in the video
If you prefer reading to watching a video, then you can read a detailed user guide here.