How do I write

I have about 2 months of free time on me rn, no studies. I'll be doing some other things obviously, can't really devote all my time to just one thing, but I had a long fic/book idea and I was planning on writing it rn.

Now I have written one short ao3 fic before and uploaded it too, but this plot idea I have does deserve a drawn out fic/book thing with substantial plot. I watched some youtube tutorials on how to write something like this, and it looks daunting there, people are spending months researching and taking physical notes filling up multiple diaries, seems like too much work for me to be able to accomplish in the time I have.

Is it actually like this though or do these youtubers make it look unattainable on purpose? Anyone who had any experience writing something with more than a few thousand words please help me out here

(also idk if this info helps but i was never a writing kid, if we had to do any sort of creative writing that was for fun and wasn't a part of the curriculum, I either made my mom write it or opted out. I really only got into writing recently because I started reading so much on ao3)