All right, kids, gather 'round and park your butts. Uncle Mac is gonna talk your ear off for a bit.
Quite a few of you write gamebooks, or want to write them and are still figuring out the best way to do it. Word on the 'net is there are a number of softwares you can use to help navigate the labyrinthine process of piecing a gamebook together. I shall refrain from commenting on those, because I am what you might call a "cantankerous old coot" and prefer not to go out of my way to learn newfangled technologies if I don't find it absolutely necessary.
Writing screenplays, for example, required that I purchase special software to write in screenplay format easily. My word processor of choice is problematic enough without my having to jury-rig its default formatting to the anal retentive degree required by the film industry. So I broke down and bought Fade-In (not a plug, but kind of a plug).
My approach to writing gamebooks was different. I didn't bother getting any computerized aides because I found a way to write and organize gamebooks without them that suits me fine. And also because I prefer to do things the hard way, apparently.
Being a decent writer is a good start to writing a good gamebook. Natch. Most of you probably figured that out already. But it's best if you also have experience as a game designer. For simpler gamebooks like CYOA, it can help you better organize the book's structure and keep it engaging; for more advanced works ranging from Interplanetary Spy to Fighting Fantasy, it's essential if you want to design effective mechanics, puzzles, and games. In either case, it can help a lot with worldbuilding, too. You'd be surprised how much easier it is to world-build with a board game, video game, or tabletop RPG prototype than with a simple novel and a ream of research notes as long as your leg.
So far I'm wearing my Captain Obvious cape. You didn't come here to listen to me prattle about writing a chapter book. Writing a chapter book isn't all that hard. The tricky part is writing a chapter book with forking paths, and then mixing them up, all without losing track of which choices lead where.
I have seen a few useful ideas for structuring a gamebook on the 'net, but when I started out writing gamebooks I found their usefulness was pretty limited. The most common method I've seen is to map the book somehow, either with a diagram, or with index cards / sticky notes.
Ghetto Scribble Diagram Method
|Plot Map for Dinah-Mite #2 and several painkillers.
The index card/sticky note method has its uses, too. Basically it's the same as the plot map technique, except it's easier to rearrange things and add new chapters. It has the added bonus of making it a lot easier to decide on chapter order: once you have all your paths sorted out, rearrange the cards into whatever order suits you and then number them.
The downsides here are 1) running out of space, which may require the purchase of a bulletin board dedicated to your gamebook projects; and 2) the page-organizing aspect only really works if you are going the CYOA route of one-page chapters, OR the Fighting Fantasy style gamebook with paragraph chapters rather than page chapters. If you want to write a basic gamebook with more involved narrative like Dragontales, the sticky notes won't help you with reorganizing pages.
Uncle Mac's Method
Finally I go through every chapter and remove the "Ch" from the header, and probably add a cute bit of art around the chapter number to indicate which act of the book the reader is in.
Some of you will no doubt find the Chapter Draft/Page Draft method standoffish. The disclaimer did mention that I like to do things the hard way. But I've found this method makes it easy to streamline cranking out chapter content. Also, by the time you're done putting it all together, it's already organized the way you want it, formatting and all. Relying on the sticky note method could lead to chapters occupying the wrong page numbers, forcing you to seek out the point where you went wrong and reorganize your chapters all over again.
Of course, the previously mentioned gamebook software probably makes all of this easier. If you don't want to bother experimenting with a new interface, though, give these methods a try and see if they gel with you a little better.
Time for bed. Uncle Mac out. Come visit anytime.