Here's how I trade.
Using Google Finance, I setup a watch list, which is basically a fake portfolio. I use the stock screener to find companies that have a market cap of at least 1B, and which have fallen more than 13% in the past 52-weeks, with a PE of 16 or less. Then I look for companies I know the name of, and look at the price's behavior over the past few years. What I'm looking for are companies that have performed relatively well over the past year or two, but which might be down for X reason. Then I add a transaction of said company, of 1 share at what I consider its last realistic highest price (meaning, if I see a high that lasted only for a short time and never happened again I don't consider that the real high, I look for something a bit more consistent).
So this allows me to track the current price VS its past high.
Here you can see the watch list sorted by Gain % using that principle. It tells me that Aeropostale and Baker Hughes would likely offer good returns since it is down 33% and 29% versus the last high I noted for them, while Activision would be a bigger risk since it is higher than the last high I noted by 2%:
This is why recently I bought BHI in the Investopedia game. I added Aeropostale to the list today actually.
The list is a bit old cause I haven't been using it much recently since I just added to existing positions for the most part, but it gives an idea of how I keep track of things. It's basically my way of easily finding what I consider undervalued stocks based on my preferences, without spending much time. I used to check stocks one by one looking at prices and all, it could take me two hours
Not saying it's a good approach, there are other factors that come into play, for example I'm much more confident in my understand of a videogame company's valuation than let's say, a tech company or defense sector company. But it worked out well for me, and I don't end up spending more time than I want looking for what to invest in. And in the end it gives you more time to think things through, rather than use that time to look for what to potentially invest in to begin with.