Years ago, long before I ever even considered that I might possibly be an ID, my English professor assigned us a paper to write a set of directions for a task of our choosing that could be successfully executed by anyone who could read English. Coincidentally, like Jerrie, I chose making a peanut butter and jelly sandwich, but for my part because I was lazy and wanted to pick a task for which it would be very easy to write the steps. Two days later I had a paper that was just shy of three pages (it was an English class, and we had to write it in prose, not instructional format), and a much deeper understanding of how much unconscious knowledge and experience we rely on to perform what we consider to be the simplest of tasks. I've never forgotten the lesson I got from writing that paper, and when I went on to tutor math and chemistry in college, and develop training material in my professional roles, I always asked myself, "what can I reasonably expect that the person reading this material or taking this course already knows, and of that knowledge, what can I safely expect that person would execute without specifically being instructed to do so.”
In the end, I’m still forced to make assumptions (as you noted, we could debate forever whether or not it’s necessary or useful to include steps to enter username and password or just a step to login), but at least I have a good framework in which to form, analyze, and act on those assumptions.
And my professor compared my paper favorably to Hemingway, so, I’ve got that going for me as well. :)