First of all, it should be clear by now that everyone has their own way of doing it and each of those ways works pretty well for everyone.
Like most things, it gets easier with experience. I don't make contact sheets at all. The main reason is that I can judge a negative better than a 35mm, 645 or 6x6 size print (which is often of less than optimum print quality). If I took the time to make a perfect contact sheet of every roll of film I shot, I wouldn't have enough time to make enlargements.
I use a cheap Canon loupe (had a couple of expensive loupes--they don't bounce very well when they are dropped, believe me ). My negatives go into a Print File page after they are dry and I judge them through the plastic. I have a portable light box I bought so long ago the brand is worn off.
What do I look for? Flaws, screw-ups and eyes that are closed, among other things. I try to find something wrong with every negative that interests me. When I find the least offensive negative, I use a Sanford Sharpie to mark it on the plastic page. I take the page to the darkroom and then I judge the negative projected on the easel. Many times, a negative that looks okay through the loupe simply looks like crap in the darkroom. A lot of times, what looked pretty good on the easel looks like crap when it comes up in the developer.
It's not a perfect method. It's subjective and trial-and-error a lot of the time.