You don't have the money for the London Philharmonic and you can't afford the rights to Air's Walkie Talkie - but you do have a small budget (or grant, or sponsor) and you've decided you want someone to score music specifically for your film.
There are several ways to go about this.
Approach a professional composer
Composers who do really good work are often good because they are very particular about their compositions. They may be used to working in isolation and, in effect, are already directing music in the same way as a director is directing a film - or a novel writer their book. It's possible that a musician will frown upon the idea of sitting in the same room with Herr Direcktor whilst seeking audience with their muse.
So how do two brilliant and yet temperamental animals work together? Well - the director has two choices; Show them the finished film or don't show the film.
The first choice can often result in the musician re-writing the director's work in musical form. This might be okay if that's what you want. But hearing the same story that is being told can sometimes come over as tautological. This doesn't always happen. Some musicians understand that music has to assist or even ultimately take back seat to story - illuminating some aspect of character or even running counterpoint to your protagonist's emotional journey.
What you don't want is the tautological retelling of the emotional story in sound. It's a horrible thing to watch.
The second method is to just sit with the composer and describe the scene in detail. There's no pressure for the composer to perform and the environment is familiar. Scoring to nothing (essentially) can result in a more natural, less pointed and unencumbered piece of music.
I recently rocked up to a composer's studio without a cut of the film (in fact - there was only a rough draft) and the composer was able to - freely - create a piece of music. I knew I wanted a 1 minute uninterrupted piece and several 10-15 second transitions. The composer enjoyed working this way and was able to score for a variety of moments in the film just by picking up on body language and brief verbal character sketches.
Admittedly, this was a series of short films and so the rigors of constructing a (non-derivative) melody for an entire feature film or TV series are quite different - but at least the filmmaker can get a taste of what is possible.
It's also important to have some boundaries when you sit in with the composer (such as segment length, rough story shape) as the session can end up, basically, becoming a jam session which can go endlessly on into the night or day.
But, as life is short, that may not actually be such a bad thing.