I wanted to try making an storygame on here about 5 years ago, but I've gotten stuck and forgot about this site till today. I still don't know what to do. I'm not exactly sure how to plan everything out, if that's at all possible. Please help me.
It depends on what type of storygame you want to write. An easy way to write a normal, branching story is to start with one path. Write the story for that entire path and go back to find place where the character could make an important, story changing decision. Then, write something for that path and continue the process.
Remember that outlining the paths is a good idea. I found Twine to be very useful with that. Think of a regular story, write that, and then make some parts split off, and some parts split off of that, and so on. Obviously, they don't have to be something completely different, and if the decision isn't important, there's really no point to make those options unless it's to explore the characters or an idea.
So yeah, third paragraph, and I'm finally going to talk about planning. Like, planning. Obviously, people do it differently. Some people take a sheet of paper and legit make a tree while others do what I described above and plan the story for the "main path". Planning means thinking about characters, subplots, back stories, conflicts, and possible endings. Treat it as you would any story you write. Think of your characters. For a smaller project that maybe only describes a total of 24 hours, you probably don't need a complete analyzation (don't think that's a word xD) of the characters, but get to now them well and how they'd react to certain things - it's super important in a storygame because the choices must be reasonable.
Another biggie: Don't make a choice immediately lead to death/lead to a page that can only lead to a page(s) that can only lead to death. Those are very annoying.
Because you're a bit unfamiliar with the system, I'd recommend not starting a huge project or one that requires a lot of scripting. Start out with something small - a branching story with an enticing plot and interesting characters. Don't avoid clichés, but don't use them too often unless you're confident that you can make unique characters. A few good characters beats many unnamed ones. For example, a storygame named Lonely Boy by Digit is a nice, short storygame that branches very well and makes use of a few good characters. Although the grammar and technical stuff of that one isn't too great, it made me want to replay it again and again until I got to every ending. The point is: A storygame doesn't have to be long to be good and rewarding. :)
Onto grammar/editing: Make sure you review those rules. Reread/replay your storygame when you can't write to see what you have so far and record errors to fix later. I think it's a good way to review the storygame, but some people just have a beta and focus on writing.
Just want to add that sometimes choices do have to lead immediately to death... or the storygame would never end.
XD Oopsies. I meant it shouldn't just be there just to be a branch 'nub'.
@Crescent: The word you're looking for is analysis, well written, by the way :)
@ The OP: I wrote an article on this, on similar lines to Crescent's thoughts but with a few software recommendations and other suggestions thrown in, feel free to go through that, you may find something helpful. There are also good resources in the Game Design segment of this article (which also highlights a lot of good content created on the site, in case you want to catch up) if you want further design ideas.
Glad to have you back :)