I love writing outlines. Outlines for CYOAS, outlines for regular stories, outlines for comic books, outlines for IF...
I probably have at least two dozen outlines for every actual finished project on my computer, but I blame that squarely on my own laziness and ADD; my outlining methodology itself is flawless, I swear.
I start with opening up a writing program (any will do) and writing down the tl:dr of the plot, just a small paragraph to cover the basics.
Then I list important characters and a few words about their personality or role.
Next comes the numbered list of major plot points. For an IF game this would basically be the rooms, puzzles, and cut scenes the player
has to get through to finish the game. For an IF game I'd also create a rough map at this point as well, or in the case of a CYOA a story map showing how all the major events link together and make sure the player comes across the important bits no matter what path they take.
Later I'll go back as the mood strikes me and fill in more about the world and any random cool ideas I'd like to add at some point. Recently I've discovered Twine is excellent for this, each character name or plot point can be linked to another page where you can type as much nerdbabble as your heart desires without it cluttering up the main summary page.
Implement the map and only the most necessary puzzles and plot points, and you have a very bare-bones but still functioning and finishable game. Everything else after that is just icing on the cake, add more detail and embellishments as the spirit moves you.