Learn about the sort of audience you wish to write for, and the genre that your script will belong to. This will also help you figure out what sort of literary tools you wish to use to enrich your script. Write with a clear understanding of the tone of your work, and have a sense of the time and space in your script. The audience should easily grasp your world and the perspective that you wish to show.
Create characters with purpose, agency and direction. It is incredibly important that a good script have characters that are interesting and relevant for an audience. Create characters who are multidimensional and layered, and who are often fraught with complexities and internal conflicts. Characters may also stand to represent certain symbols in society (i.e., the moral compass) but should not become cliches. Dialogue plays an important part in character development and in establishing various relationships between characters. Character conflict is an important part of every script, and a writer should have an understanding of the conflicts expressed in the script and how this relates to the overall theme of the script.
Plan each scene with thought and purpose, and without any unnecessary text. Avoid scenes in the script that do not serve a purpose of some sort. The dialogue between characters should stay relevant in each scene and propel the story forward. A good script will have at least one strong climax point, usually toward the end, with tensions and conflicts building, and then being resolved after the climax itself.
Finish a script in a way that is satisfactory or aligned with the purpose, theme or ideology of the play. A good script will either resolve outstanding issues and conflicts in the play, or purposefully leave these unresolved to make a point. A good ending will leave the viewer in deep thought about the overall performance and about the topics explored in the script. There should be no obvious loose ends by the end of the script, unless, of course, this is done purposefully.