Imo, it's about figuring out where you have an advantage over the Wall Street big shots.
I like to focus on small and mid cap companies in industries I know well. It helps to understand the business as well as what metrics, financial and non financial, are really important. I try to avoid complex legal and capital structures as well as the consumer goods/retail stocks that everyone knows about. I figure if 99% of America and 90% of Wall Street has never heard of the company, I'm a lot more likely to beat the market to a correct valuation. Right now, for example, I think I've tagged a couple companies that will initiate dividends soon but they are still priced like they won't.
Sometimes I start with screens, other times I work backward from industry/ sector trends to companies that are poised to exploit those trends.