Nothing pleases me more than when I ruin a guy's orgasm. I have written much on the subject here, but I would like to give you some quick tips on doing it yourself.
If you enjoy edging your guy, that is bringing him as close to an orgasm as you can and then stopping (or pausing) for a moment so he can relax and back off a bit, then you are already half way there. My first tip is try it by edging. It doesn't matter how many times you stroke his cock, as long as you stop for a minute or more to let him back off from the edge. Even if he has not quite reached it, you have been successful and he will enjoy it.
If you DO manage to get him right up to the edge, then you have reached that perfect point. The point where, just one more stroke (or two) will send him over the edge, shooting his ejaculate all over the place. So what you need to do, is pay close attention to everything he does. The more you are able to repeat the process, the better you will get at determining exactly when to stop.
Here's a little hint: keep an eye on his hips. If he tenses his hip muscles (raises up slightly if laying down), he may be getting very close and it's a good time to back off. If he lowers his hips below the normal resting position, look out. He may be ready to shoot and it may even be too late.
The key here is practice. Practice makes perfect. Practice is fun! He will love it, and if you don't stop in time to watch his ejaculate simply drip from his cock, well, there is always next time. Like I said, practice is fun. If you stop too soon, keep trying. And remember, if you succeed, you can try again right away. If you fail and he cums hard enough to “shoot”, then you will need to wait a day (or at least a few hours) to allow him to recuperate.
So have fun, and don't stress out if you fail the first few times out. You will eventually get the hang of it and you will learn exactly what to watch for. Every man is different, so I can't give you all the specific signs to watch for, you'll have to figure them out for yourself. But don't forget to have fun doing it.