Go to the freestyle level. Get your boost all the way up. Start going around the course to get speed. If you choose to use the loop as your take off, you need to boost all the way around it. Another good ramp to use is the one that is sort of under an elevated ramp; you will not need boost. To do many flips, choose your ramp and get speed. When you are about 20 feet away from the take off, hold Analog-stick Down. Keep it held and when you get to the peak of the take off, you will do many flips.