Imagine a movie running in your head… just like reading a book when a mental projector illustrates the action as you turn the pages. (At least it does in my case.)

The same principle applies in script-writing – or screenplay creation (or treatment) as it’s usually known when you add scene sequence and shot-framing, sound effects and background music, dialogue, graphics, and any sub-titles that are needed to help viewers follow the storyline.
As with the opening page of a book, or the intro para in a news story, first impressions are everything. The visuals must catch the viewers’ attention straight away – and hold them waiting for more.
Again, ‘tone of voice’ is as important as it is in writing heavyweight corporate brochures or chatty social media posts. Do you take a redtop tabloid ‘shock, horror’ approach to presenting the front-page splash or a more restrained but equally irresistible David Attenborough style of wildlife documentary?

The subject matter will always be the best guide to that – and if a touch of levity can be introduced to the proceeding. Always a bonus, but not always appropriate. You be the judge, and if in doubt seek a second opinion. From a colleague or a trusted friend – never from the client! As with all copy, your treatment will benefit from a second opinion anyway.
Once you’ve got that sorted, keep seeing the story in your mind’s eye. Remember that the pictures always come first: the script, music, sound effects etc are complementary.
For example, I sometimes find myself dozing on the sofa with some programme on the telly burbling away in the background. Something will catch my attention and I will listen, never opening my eyes. This could just as well have been a radio documentary for all the good the pictures were doing.
Perhaps they were very visually powerful, in which case let them tell the story. Readers can see for themselves and don’t need to be told what they’re looking at, unless the words are adding background or detail that’s not provided by the visuals.
The same applies to sound effects and especially background music. Some video-makers seem convinced that a blaring soundtrack is absolutely indispensable, at max volume and at every opportunity irrespective of appropriateness to the subject matter.
‘Bohemian Rhapsody’ for an obituary tribute? Lift muzak for a high-tech product launch? No, the SFX and the music are supplementary and should be unobtrusively contributing to understanding and/or entertainment, not dominating the proceedings.
Writing the script itself follows the same rules as for all writing: keep the language clear and lucid, avoiding lengthy sentences that will need a respirator on stand-by for the person doing the voice-over. Reduce descriptions to understandable dimensions that people can relate to. They can visualize10 rugby pitches but 200,000 square metres? How loud is 1,000 decibels – maybe 10 full-size airliners taking off at the same time? Or 10 million kilometres of fishing line – how many times to the moon and back? Insert illustrative graphics to help.
Finally, and it can’t be repeated too often: let the pictures do the talking. Think of the screenplay as a whole, where the script is an essential component but just one ingredient in the recipe to produce a final product that will have viewers licking their lips. No ‘smellies’ yet to add to the anticipation and enjoyment, but you still have ample resources at your disposal to create a treatment that will have them coming back for more.