I don't really have experience with issue format, so this advice might be a bit general.
Remember that with purely written stories, you can have a lot more vagueness. Stuff like time, when things happen, is going to be a lot more vague than in say, a movie.
Time vagueness is probably present in comics, but less so since readers will probably fill in the transitions between panels with their imagination.
The way things actually look can be surprisingly vague, given the prevalence of description. Note that you don't have to describe a character perfectly, describing each piece of their outfit and features will take a while if you go overboard. Same thing goes for the location the scene is in, it can be vaguely described.
In comics, since you draw things, character outfits and the locations will be a lot more concrete, less imagination in that regard on the readers part.
Granted, all this isn't a rule, it is more of a common path (from what I gather). Depending on your style, you can describe characters looks a whole lot, and you can describe the locations a lot. Heck, even with time you can get more specific (although I feel this will be a bit more difficult to achieve compared to the other two).
Anyway, the main point I'm trying to make, I suppose, is that you should keep in mind that different mediums have different strengths. Try to play to these strengths, or at least keep them in mind, it might help you when trying to figure out how to go about translating your ideas into writing.
If you have the spare time, try taking a look at a movie adaptation of a book, and then read the book (or the other way around), and play spot the difference.
Obviously comic to novel is going to be different, but it might help you get over the issue with converting between two mediums.
As was suggested, give writing without planning a go. You don't necessarily have to try and do this with your main ideas, but just writing anything might help you figure out ways to plan better in the future.
Writing/art collaborations is a thread by CrashAndDive that you might want to give a shot. Try asking for a prompt, and writing something for it. If you are the first one who does this you are pretty much guaranteed a drawing!
Why do I suggest this? Well seeing the way people translate your writing into a drawing could be interesting, specially since you might already have a good idea of what would look nice given your prior experience. It should also highlight the vagueness I mentioned earlier, as the drawer will have to use their own imagination to do a lot of the work (assuming it is more than just a sketch).
Another thing you can try and do is to write in issue format without the pictures, and then doing paragraph breaks for the different panels. Then just add in some description for what would be seen in the pictures if they were there.
Have you heard the saying, a picture tells a thousand words? Keep that in mind, since you probably don't want to be writing a thousand words each time (since the scene might not change much, or it might end up ruining the pacing). This sort of relates to the vagueness of description I mentioned earlier, it is a useful skill to be able to make use of the reader's imagination. Obviously but, do what works for you, as there really are many ways to go about writing.
I should also mention, don't be overly critical on yourself either, perfect writing without redrafts/proofreading is pretty much impossible the first time around. Also, while it is important to spot the mistakes in your writing, you don't want to forget about the good parts. Not to mention that if you constantly focus on the negative, you'll probably find motivation harder to find as well, which is never fun.
Another thing, I suggest starting on the smaller end, since CYOAs really aren't the easiest thing to write. They have all the hurdles of regular stories, but then you add in a bunch of CYOA specific ones. This is another reason to try the thread I linked above, as it won't be a CYOA (meaning it should be considerably easier to write).
TL;DR
There are many ways to go about writing, above is just my advice based on how I understood some things.
Remember to look for the good in your writing, can help motivation, is also a good thing to do.
Play to the strengths of the medium (ie. vagueness and using the readers imagination, but there is a lot more to it than just this).