Okay, so there’s a ton of advice out there to help craft a good script, but if you want to write a truly HORRIBLE screenplay, one that's guaranteed to be shredded by every unfortunate soul that comes upon it, then follow these top ten essentials:
(1) Develop no plot as it just gets in the way of artistic discovery.
(2) Create boring and forgettable characters that all sound the same.
(3) Never clarify any goals for the multiple protagonists of the artistic discovery.
(4) Do not waste time with research and assume whatever you think you know is correct.
(5) Look for any opportunity to use prose by writing thoughts we can’t see.
(6) Always craft long-winded action paragraphs at least twenty lines each.
(7) Make characters do or say things they wouldn’t just because you need it so.
(8) Make all scenes start earlier than necessary and stop before any conflict occurs.
(9) Only write dialogue with monologues that is forced and unbelievable.
(10) And NEVER, EVER use proper screenwriting software because the fun of reading is to try to decipher a format only you can understand.