Hmmm, all too often the starting point is a trip to a fast food restaurant.
Sometime during the meal I will have an idea, so I take the tray liner and turn it upside down. Then I pull out a pencil and either jot down the first notes, or a quick map.
I still have a stupidly large number of those tray liners.
If the placemat still makes sense a few hours later then I do a flowchart, or I pull out the Campaign Planners from Ronin Arts. If you have difficulty organizing things then this series might make a good starting point.
Campaign Planner I,
Campaign Planner II,
Campaign Planner III. There is also a Campaign Planner Deluxe, but the individual sets are better - they have fillable forms, and can be filled out on a computer and printed out.
I am not exactly unbiased in my opinion of these sheets - my name is hidden somewhere in the credits and acknowledgments on two of them.

They help a great deal in taming the pile of scraps and tray liners that I would otherwise produce. Phil Reed did a very nice job with these. There are a few similar sets by other companies, including one specific to Pathfinder.
You may want to dig around and see if you can find the very, very nice Storyteller's Notebook that used to be available free on RPGSheets.com It includes some sheets that are designed for creating flowcharts for adventures and campaigns. Unfortunately, a few years back RPG Sheets started, umm, sucking.

They got rid of a whole bunch of sheets, some of which were better than much of what they kept. The Storyteller's Notebook was among them.
Another sheet that is useful for quick and simple flowcharts is the Chase sheet from the very, very old Dungeon Master's Design Kit from TSR. I want to say that it dates back to 1988 or so. The Chase sheet is also amazingly useful for designing, umm, chases.

Essentially a series of circles with lines for labels and boxes around each circle that can be used for containing drawn arrows connecting the circles. Twenty years later I still use that sheet. (And another reason to be annoyed with the lack of legal TSR PDFs. I am so glad that I bought mine when you could still get them from Drivethru.)
The Auld Grump