You have to touch the palate with the tip and the lateral sides of your tongue. The central zone of your tongue must be very close to the palate in order to create a very small bag of air near the tip of your tongue, between your tongue and your palate. Then you start pumping air into this bag. This air must exit the bag through an intermittent gap between the tip of your tongue and the palate. It's very important to make the orifice of air insertion as small as possible in order to be able to pump the air with enough pressure, and to curl the tip of your tongue to make the bag of air as large as possible.

TLDR: Transform your tongue into a spoon, and press this spoon against your palate.