Planning
Short Answer
Plan your story whatever way you normally would (since there are many ways to go about planning), if you don't have a preferred style yet, experiment. Trying to figure out the basics of the story is generally a good idea, since having some idea of what you want to write can help you figure out what sort of branching style would work best. Here is a link regarding some styles. At this stage, you should be able to get a good enough idea of how you'll be handling the choices/branching, which should help with that aspect of cyoas. A good piece of advice often given is to start small, branching stories can get out of hand very easily, and it can be hard to grasp if you haven't done it much before.
Things to aim for during the planning stage depend on how much planning you prefer; gardener/discovery vs architect/outlining. Think of it as a spectrum, since thinking of yourself as just one or the other is generally not nuanced enough. I will point out this excerpt from the above link's conclusion: write as much as possible, and figure what suits you!
EDIT -- Basically, do you like to plan things out then write them, or figure things out as you write. Again, generally it is a mix, but I find it interesting to think about (and I bring it up rather often).-- EDIT END
Remember that proofreading/drafts/rewriting are all great as well, they can push writing to become really good. Very few professional authors just write something first go around and have it published, and I'm pretty sure they have editors. Now, I'm pretty sure most of us here are amateur writers or hobbyists (or whatever you wanna call it), and it isn't like people will be expecting perfection, (reading some higher rated storygames can give you a good idea of what people like), so don't fret too much and try to just write something. Things can almost always be improved, and one of the commonly hardest parts is just writing.
With that said, I'll mention that you don't want to get stuck planning forever, but there are definitely competent writers here who do spend a lot of time planning, so just:
- Remember that there is (usually) no rush.
- If lots of planning is what you prefer, go for it, it can work.
No-go Zones
For writing... nothing really, but certain things might be widely viewed as illegal or immoral, but handling more mature/controversial topics is fine, just have some tact and understand that there might be some people who really don't want to read it (so I'd say ensure it is properly labeled, on this site that'd be setting the appropriate maturity rating).
For the site:
- Outright porn (including NSFW images).
- Warrior Cats (due to a troubled history). This is a certain fan-fiction.
Site Guidelines, also: Minimum Storygame Standards.
Getting Black Out Drunk
I'd say it depends on where (country), but I'm pretty sure this is an unpleasant experience and should thus be avoided.
TL;DR
There are many ways to go about planning, try out doing a lot, a little, none, and everything else to find what works for you. Generally starting small is good, since branching narrative stories can get out of hand very quickly. You still want to figure out the general things as well when planning, broadly being: what type of story do you want to write? Then go from there, depending on your answer. (Most stories will have some sort of event/conflict/contrast that is central, with CYOAs you can obviously have multiples depending on the paths, but ultimately there is lots of freedom [as with all writing], I'll also mention thinking about themes might be helpful).
Finally, I'd say that you mustn't forget to start writing at some stage, since while planning a lot is fine, it won't produce a story all by itself. Find what works for you, and feel free to drop by in the Writing Workshop if you end up wanting more specific feedback or thoughts on stuff, or just sharing unfinished writing.
Closing Question for All
For a question of my own, tell me, how would you explain planning in general (but being more specific than I was with 'plan your story whatever way you normally would', i.e. what is a normal way to plan a story?).
P.S. I feel there is a lot more to mention (thus the 'short answer' subheading), but hopefully you get something useful out of all that is here atm. Also, as a disclaimer, take what I wrote with with a grain of salt, since you can technically do whatever you want and still potentially end up with something you are happy with (but I did try to give mostly applicable stuff given the broadness of the question). Oh, also, if there is something I missed or you got another question, feel free to ask (either I or someone else should answer, or both!).
EDIT --P.P.S. Oh, I didn't mention the advanced editor and how it can help you with certain things... so I'll just say that you can utilize the computer based medium to your advantage with certain things, such as tracking a variable to have a delayed reaction to a choice (i.e. it occurs at a latter page rather than straight after the choice) along with modifying on page text based on variables.
This doesn't matter too much, but just know it is possible, but not necessary. Certain things will be super tedious without it, but if that happens you'll probably notice and ask for help at which stage the specifics can be explained (assuming you don't figure it out yourself).-- EDIT END