The initial question of where to start is answerable by looking at that example. The idea for last Sunday was to organize my top reading from 2021, and I logically started with the first book I finished reading that also made my 2021 shortlist. I set some basic parameters for myself - such as including a short blurb on how I saw the book in hindsight - then I began writing. This might sound almost too simple, but I think it's the right approach - if you make things too complicated at the beginning, then you make writing harder than it needs to be, and you make it less likely that you'll ever start writing.
The next question is probably the most interesting of the three, but also the toughest one for which to provide a definitive answer. What happens along the way? The short version is that it probably varies not just by writer, but also by topic. In writing last Sunday's post, there was a moment where I admitted to myself that despite considering A Room of One's Own one of my top reads from 2021, I didn't really have a clear reason for why I included it on the list. When I reviewed my original post about it in April, I recognized that I had already avoided the same question on the prior occasion. The process of discovering that I didn't know what I was writing about forced some deeper thinking, which I think is the crux of this process. The final outcome was essentially a hypothesis, which I was entirely unaware of when I had started - books written about the specific problems of a time can still resonate with me after some creative reflection. This came about because writing forced a confession that I didn't know what I was writing about after all; the post was impossible to write without new ideas.
The third and final question sounds good on paper, but it's a poor question because a little extra thinking reveals that there can only be one answer, which in a way is also no answer. What happens to the original idea? It is very simple - I'll either write about it, or I won't, and we just have to wait and see. It comes down to whether, having wandered off the original path to explore a new journey, I feel there is a decent reason to return to the original plan. There is no definitive process to govern the situation because the situation itself was a result of proceeding without a governing process - I started with a plan, yet retained the flexibility to reevaluate it along the way. I think that's the best way to describe this lesson - if you want to have your best idea, you have to accept that you need to start with a decent idea, then make it a better idea; your best writing is a result of starting on a path toward good writing, then looking for signs of the surprising detours that make it better writing.
