The so-called spotting session is the meeting between director and composer where both watch the movie together. Usually, this is a great chance to find a concept and ideas for the film music in a mutual way and to avoid any misunderstandings in the actual working process.
You as the director should take the lead in this. Also, often the composer gets to see the fine cut of the movie for the first time in this situation so it is also a great chance for you to get a bit of first reaction feedback by someone who isn’t biased or work-blind regarding the movie yet.
If there are shots still missing or essential cgi elements of the images are not visible yet, you should of course explain in these scene what will be seen there when the movie is done. The most important thing to do in this meeting is to give the composer the chance to get a thorough understanding of the movie, the plot and your filmic intentions so he/she can think about how it is possible to contribute musically to this vision.
In most cases, the procedure is to watch the movie once in complete and then work your way through it scene wise, discussing where there might be music and what the music should do. It is very important to remain open to ideas by the composer as he/she might have a view on certain things that you don’t but which could actually push the movie further. Things like tempo, mood, thematic connections etc. need to be discussed during that and usually the composer or an assistant will make notes about the decisions and it of course would be clever for you to make notes yourself so you have a reminder when there might be a conversation between you and the composer later on.
It is not uncommon for this procedure to take at least one full day, if not more.
Due to budget constraints and long distance working relationships, spotting session have become more rare recently and are either being held over skype or by email which of course is also possible. However, you should not underestimate the creativity that can come up in a situation where you and the composer watch the movie together in a darkened room so whenever your budget and time allows for a spotting session, you should try to make it possible in order to get the most creative musical result for your movie.