There are two ways you can position yourself to begin the actual writing of your first draft.
One is to review all your pre-draft exploratory work one more time–any character work, backstory timelines, plot outlining and anything else you’ve explored regarding your developing story–and then, leaving all this material handy for reference, simply plunge in with page one and see where it leads knowing you can refer to any or all of this developmental material at any time.
The other approach is to put all your prep work away and don’t review it at all, the theory being that you’ve done the pre-draft work and thinking, so it’s up there in your head somewhere for guidance if and when you get stuck in the actual writing of your draft. The thinking here is that this allows your characters more freedom to do and say what they want in the moment and reduces the temptation to force them into a predetermined mold.
Of course, there’s not one best way to get started. You simply have to discover a procedure that you feel comfortable with, that puts you at ease. If all your prep work makes you nervous, get it off the table and file it away. If it gives you a sense of security at the starting gate, keep it there in front of you. Just always remember that first drafts often have a way of taking on a life of their own fairly quickly and you should give yourself permission to explore where your characters might take you regardless of any predetermined route you’ve worked out for them.
Ultimately, no one but you really cares how you get started with the actual writing of your script. People are only interested in how good the finished product is. As the late, great playwright Romulus Linney told me about getting started with his first drafts: “I have mulled it over and thought about it this way and that way, and there is always a moment when you say, ‘Okay, for better or worse, here we go.'”