I’m far from professional, but have done a bit of research and “played” a bit with the art. One technique that seems common in response to your question about outlines and synopsyses is to write down broad ideas on index cards. So, you have several major sub-stories running through your screenplay. Write each sub-story as a series of cards with summaries of major points of the sub-story. Then you can mix your deck of cards to interweave your stories until they are as you like. Using the deck order you write the actual screenplay.
Before you write (or as you write), it’s also valuable to write a description of each character, including such things as (for example) name, age, gender, physical appearance, personality, likes, dislikes, strengths, weaknesses, basic goal in the story, desires, traumas, etc. This should assist in making sure your characters have life and are consistant.
Same thing locations: list each place, a description, and a function (the villains hideaway, the place the lovers meet, etc.)
It’s also valuable to write down a sort of a summary, including such things as genre (science fiction, romance, fantasy, etc.), the basic concept of the story, where and when the story is set, theme or subject that underlines the story (forbidden love, revenge, coming of age, etc.), the outcome (hero gets girl, main character dies tragically, etc.), the target audience.
As I say, I have no professional experience or even training. But these seem to be standard techniques, and have helped me in my amateur efforts.