My female friends tell me that if a guy comes up to talk to them - your body language gives away that you're interested whether you know it or not. You don't have to say it. In fact, it can be awkward if you do say it right away without justifying it (e.g. 'you are hot' v. 'i like your aloofness').
Even the nerdiest person has thousands of years of built-in intuition that trumps all conscious analysis.
The hardest step, for me, is just putting yourself out there.
If you try to start a conversation in fight-or-flight mode, your body language will betray you, and it will either end quickly or carry on awkwardly.