Cars are set up so that rear brake engages just split second before the front when you push the pedal. Also, front brakes are given a lot more power.

So perfect setup would be to start with the rear (weight transfer). But that wold make more sense on a bike with suspension. On a rigid road bike, not that much.
Then, split second later go with the front, gradually increasing the pressure, sqeezing the lever. Not too slow, but not in a jerking motion. More like sqeezing an orange. Quick, firm, but still gradual. As you press the brake weight is transfered to the fron wheel. That gives it more traction, so you can press further - all to the point of tyre begining to scrub off and slip. With experience, you can learn to loosen the brake just a little bit, or stop pressing harder, just before that happens. On motorcycle, I had some cool Continental tyres that would make a really loud sqeel before totaly letting go - so you could always tell the limit.
While doing the afore mentioned with the front - you can just feather the rear, to slow the rear wheel a bit, but not let it skidd. On a road bike, this is very hard, so I set my rear brakes so they have less stopping power and it is harder to press them too hard.

This all applies to hard paved surfaces, without (much) debris. On really steep downhills, rear wheel has even less braking power.

Oh, this is very important, vital: put your weight all the way back before hard braking (if possible, if not then during), and hold elbows bent, arms loose. Make sure to keep a nice loose hold on the bars. Firm so you don't fly over the bars, or let your body move forward, putting too much weight on the front, but still not a death grip, locking one. Your arms and legs should work as shock absorbers. Locking your elbows, or holding too tight will make front wheel skid much earlier, or make you fly over the bars.

Practice panic stops. Place markers, find a free part of pavement and practice. At different speeds. It is worht it. See how bicycle behaves at greater speeds.