That's a good question.
I suspect everyone's different. There are a slew of books that describe how to get organized, that have templates on plot outline, that describe what story beats you need to hit on which approximate page of the script, etc. I think they're helpful to many people because they condense and sift through the structure and form of hundreds of highly successful screenplays for generalizations that can be applied. To fill a minimum of 90 pages of script with individual scenes and multiple character interactions that coalesce into a good story is a complex undertaking. That means lots of planning ahead. A story board divided into three acts with 3X5 cards pinned to it, each representing major scenes, is a good way to stand back and visualize the structure of your story. It's easy to replace the cards, or add cards, or rearrange scenes by simply moving the cards. I use this method myself. But again, everyone is different.
On the other hand, I've read interviews with very successful pro screenwriters who insist they start with a solid idea, theme or logline and then almost immediately sit down, write "FADE IN" and begin to bang away. They work out the story as they write. Or so they say. I think they're being a little bit disingenuous, however. I suspect that while it's true that they may not have detailed notes or a plot outline written down on paper, they DO have it fairly well sorted out in their heads.
As I've mentioned elsewhere, script guru Robert McKee claims that AT LEAST 75% or more of a screenwriter's effort and time must be spent on story design, and the rest on actually sitting at a keyboard and writing dialogue and action and sluglines.
He may be right or wrong, but since the wittiest, most brilliant dialogue or the most succinct and exciting action descriptions in the service of a suck-ass story is basically time wasted, I tend to agree with him.
best,
-Charles