Pick an index fund. Boom, you're diversified. Now, keep an eye on developments, read newspapers (I recommend FT, NYT, and Economist), something will start to tickle your fancy. When you feel like something looks interesting or resonates with you or your background, dig into it. Allocate, if you can, at least 2 full days, for example a weekend. Do on-the-side reading beforehand, gathering all sorts of material (reports, statements, articles, interviews, customer opinions, etc.), just read superficially, the goal is to get an idea and to narrow the sources down so you can do your final research in the 2 pre-allocated days. When after a while, your interest is still there or better yet growing as you find out more, and only then, pull the trigger and invest those 2 days. If you can't stomach the risk at the end of them, forget about the opportunity. If you feel good about it, pull the trigger again and take a position.
Start with a small position, shifting some of your index fund into the stock. Maintain and see how it feels to have the position "on". If it feels good, take more (long _or_ short). If it feels wrong, get out, whether you're ahead or behind.
Forget all about diversification too. Your goal is to iterate this until you have a portfolio of perhaps 4, 5, 6 positions. You are substituting "diversification" (you can't be better diversified than a broad index fund) with knowledge or insight or whatever you call it.
Just my 2 cents (har har, I know... ;)