I think about it, mull it over. Writing it down helps me formulate and clarify my thoughts.

I don't read reviews or articles until after I've written mine. If I have missed something well that's too bad. I don't really expect anyone to read my reviews, anyway. They disappear in amongst the hundreds or thousands of other reviews.

To be fair, that's the way my brain works anyway. I have a Humanities degree and I am accustomed to thinking by writing.