Step 1: send out a few feelers, try to find out your market worth. Call this $X.
If $X <= $YOUR_PAY + 3% (margin of error), STOP. You are not above the median and you do not deserve a raise. (Most people think they are above average. Almost 50% of them are wrong.)
Step 2: start interviewing. Ideally get an offer above your pay (but not necessarily as large as $X).
Step 3: have a private conversation with your boss. Tell him you'd like more money, and you feel you deserve at least $X.
If he gives you the raise, declare victory and decline your external offer (unless you like it better). If he refuses or gives you a raise that is insufficient, either a) point out your external offer or b) give 2 weeks notice and accept your external offer.