Scratch Roll = poorly executed Press Roll...
Seriously, though, I'll assume you mean a Press Roll. These are executed by using the same downward wrist stroke as you would use for a single stroke, but then applying a certain amount of "pressure" with the fingers. The Middle Finger is key, according to me and Joe Morello.
The "pressure" is applied in order to cause the stick to bounce off the head multiple times in short succession. The number, speed and texture of the multiple bounces will vary with the amount/type of "pressure" applied, and according to the technique of the player. As such the Press Roll may be played in a very "scratchy" or a very legato way.
To start, however, simply drop the stick on the head and let it bounce. At this point, don't worry about the number of bounces or the texture of the roll. Just get used to loosely dropping the stick on the head and letting it bounce. Ensure that you're getting a similar sound from both hands. Once you've got that down, drop the stick and then experiment with applying "pressure" upward with your middle finger in order to tighten-up and control the bounces. Practice this slowly - hand-to-hand - and don't worry if the rolls aren't even and connected at this point. Just get the feeling of loose wrist strokes followed by the pressure from the middle finger. As you get this together, you can speed up your hands and close the roll.
Another technique which I periodically apply is to use a slightly angular or circular motion, dragging the sticks across the head, rather than straight up and down. That said, I highly recommend you learn the first type of roll at first rather than falling back on the "whip cream" method which can be easier to execute at first. The benefits of learning the fine muscle control needed for the first type of roll cannot be discounted.
And last, but certainly not least, find a teacher who can show you all of this in person, watch you do it, and correct your mistakes.
