A Shuffle is simply extending the boost from either nitrous which is gained from rings or stunts; or a boost pad. In order to execute a shuffle you simple press the handbrake while holding the gas and slide left or right, once you back out of that slide you will start to lose speed. You can either do an Assisted Shuffle which is lining yourself against a wall for assistance, or a Unassisted shuffle which is simply just sliding wherever. One of the best places to practice Shuffling is either Vapor City and Shuffling against the loop's walls, or Satellite City which is good for both Assisted and Unassisted Shuffles. I advise first practicing Assisted Shuffles until comfortable before moving to Unassisted.