Once you have the initial story or the book that you need to build the screenplay on, you need to figure about the key characters around whom the story revolves, these are generally actors who have a line or two. Once you have the Primary, Secondary and support characters sorted out, you need to start visualizing the book as it was a scene. A lot of cues to the background location, dress and time of day will be in the book itself.
The opening of the series should have punch and should hook people ideally keep a hook at the beginning and the end of each series. Hooks are great attention grabbers, you can start an initial scene with two couples making love as people peer at them in the act through their bedroom window. Or maybe a man in a room covered in blood with a knife, as he moves we see a dead body stabbed to death in front of him. These are drastic visuals that get the attention of the audience.
You need to write the scene first, witch has cues on which the director and cinematographer can work on. It should be a paragraph or so that explains the setting, reason, and situation of the scene. You need to see the characters covers under certain settings and under a certain mood. Also built-in it should be how the actors and characters are feeling and how the camera will move from one screen to another or for that matter form one actor to another. These are cues for the cinematographer.
Then you fill in the dialogues, again look for punches more descriptive words, use slags of local lingo depending where the scene or the story is set in. Use sharp words and experiment with street dialects also. Keep it humorous, jokes, wisecracks, oddball stories, daily masala there is a lot of inspiration out there. But keep the flavor local and relevant. Try and pick as much from the book itself, but you can add some of your jokes and punches also. The screenplay writer has that liberty and can come up with good suggestions and improvements in the initial story.
You need to keep the duration of the screenplay in mind 20 min to 30 min are the ideal lengths for web series screenplays.