As a trailer editor, it's normal for me to focus on individual pieces: dialogue, shots, transitions, graphics, text, sound effects, and music. A trailer editor's skill is ability to cut something into pieces, and in many cases make something new by disregarding how they work in the context of the greater work.
The problem is: How do you even begin to bring order to these small pieces?
This is an imperfect analogy, but think about assembling a jigsaw puzzle. When the pieces are poured out of the box, people don't just pick a piece, and start looking for its adjacent piece. Typically, the first step is to sort the pieces into piles whether it's by the edge pieces or by similar groups of colors. This is essentially what I do with all the clips I start a project with. Let's start with dialogue.
I discussed some of these ideas in my post about Red Dead Redemption 2's trailer and paper edits, and my post about a trailer for Star Trek: Discovery, but I'm going to spell out some concrete steps for my process.
1. Select dialogue which includes story exposition, character building moments, plot events, and action.
Even though I've done this for years and years, when selecting dialogue I still find myself thinking: "Does this sound like a line I've heard in a movie trailer?" Generally, the less a line of dialogue affects the plot, the less likely I am to select it (e.g. Yoda in Empire Strikes Back saying: "How you get so big eating food of this kind?)
2. Take the best lines of dialogue, and write a summary of the story it illustrates (it's okay to be abstract).
Dialogue: "Great shot kid, that was one in a million!"
Summary: Luke made an amazing shot.
Dialogue: "The Force will be with you, always."
Summary: The Force protects people
Dialogue: "Tear this ship apart until you've found those plans..."
Summary: Darth Vader is looking for plans, and is willing to be violent to find them.
3. Put the summary chunks into an order which creates the story you want to tell for the trailer.
This is the hard part, because deciding the order of events is the first step of the process which requires creative thinking. Editing the dialogue first using the summaries is much easier than editing based on the dialogue itself because you can create sentences which read somewhat like a synopsis.
"Ship is attacked, Darth Vader is looking for plans, and is willing to be violent to get them. Young boy meets droids who have plans, Young boy finds Old Man and discovers what plans are for, Old man hires pilot, they go on dangerous mission, there's lots of harrowing action. They may or may not succeed."
4. Put the corresponding dialogue from each story chunk in the order from step #3. Then remove redundant lines, or lines which don't flow with adjacent dialogue.
This part is even harder because you're deciding what the final lines of dialogue will be. I always save this raw version just in case I get second thoughts and decide to use a different line to illustrate a particular story point. The added benefit of this process is how easy it is to come up with different story structures for the trailer since it works in chunks, not details.
Now let's look at how this process can be applied to visuals.
When visuals are sorted like this, instead of going through shot by shot, I can say to myself: "I need a heartfelt moment for this section to show a moment of relief" or "I need an explosion to make this montage more exciting."
I also do this with music, because music is composed to evoke distinct emotions, energy and moods. On a recent job I went through all the available music I might use and labeled each section according to how they made me feel.
Some descriptors included:
MAGICAL, CHILL AMBIENCE
DARK DANGEROUS ALIEN ALLEY
COLD ALIEN SEWER
CAUTIOUSLY OPTIMISTIC DEEP SEA
I just thought of whatever image immediately sprung to mind when I listened to each music cue, which meant whenever I looked back at what was available, I instantly remembered the feel, and was able to quickly decide what would work for that trailer.
Sound effects can be thought of in this way too. If you've ever listened to sound effects libraries for trailers, you know they're tailored to very specific sorts of moments you might find in trailers (sadly, if you listen to them you also realize 90% of the sounds are catered towards action, science-fiction, and horror)
If you have an extensive library of sound effects, it's worth becoming familiar with the identity of each, so you know where to go when you're trying to match a moment in a trailer. Categories could include: Horror movie drones, cartoony sound effects, magical whooshes, Transformers fight scenes. These help when I need to find the right sound for a shot for something very specific like an intimidating space ship.
I often lament how it's much harder to see early drafts of a trailer as a "sketch" of a finished piece rather than just a bad version of the finished piece, but spending more time either on paper or typing and sorting in a text file is a great way to create early drafts before clips get put into a timeline.
So if you're making a trailer, try to first work in chunks; it's much faster and easier to get anything done at all, and then later you can chip away until the the refined piece is finished.