Many screenwriters find index cards (aka notecards, note cards, or flash cards) valuable in both the outlining and pitching processes. They are a great way for visualizing story, and they are non-linear, so they can be moved and shuffled easily. Plus they come in different colors, which can be a big help with organization.
When using index cards for outlining, some writers get detailed with their cards, making one for each scene. Blake Snyder’s Save the Cat! proposes using index cards to get a handle on what he sees as a movie’s 40 major beats (10 each in acts 1, 2A, 2B, and 3), while other screenwriters simply use them to keep track of the most absolutely vital turning points in the plot.
Writers often have their own ways of implementing index card color coordination. Even with the same writer, color-based tracking systems can vary from movie to movie based on what would benefit from it most. Colors can be used for visualizing how scenes work emotionally, how they work tonally, which characters are featured in which scenes, scene locations, time of day, or for tracking any other variable element.
In a 2010 blog post, screenwriter John August offers these index card tips:
Keep it short. Maximum seven words per card.
A card represents a story point, be it a scene or a sequence. You don’t need a card for every little thing.
Keep cards general enough that they can be rearranged. (“Battle in swamp” rather than “Final showdown”)
Horizontal (a table or counter) often works better than a vertical (a corkboard).
Post-It notes make good alternative index cards.
Consider a letter code for which characters are featured in the sequence. Helpful for figuring out who’s missing.
Most movies can be summarized in less than 50 cards.
Cards are cheap. Don’t hesitate to rework them.
Consider a second color for action sequences. Helps show the pacing.
Write big. You want to be able to read them from a distance.