There are two ways to do the double stroke roll. There is the bounce back – where you hit the drum once and allow your stick to bounce back. Or you can use your wrists – where you physically hit the drum twice with each hand. Both are good techniques with plusses for each, and should be practiced equally. The trick to mastering the double stroke roll is to try and make both strokes and both hands, sound the same. Close your eyes and listen, do they sound the same?
To figure out the double stroke roll, start out slow and with always play with a metronome. Really try and get a feel for the stick. Simply hit your Practice Pad, and allow your drum stick to bounce a few times. This will get you used to the feel of your stick bouncing back. For using your wrists, again, start out slow. Simply double the single stroke roll, alternate each hand hitting the pad twice.
once you can master that start playing single strokes for one bar and then double strokes for the next bar. repeat these two bars continuosly and try to ensure that your doubles and your singles sound the same. practice this exercise and build it up to speed. you can also add paradiddles to your exercise for variation and independence purposes.