touch the top jaw right behind your teeth with the tip of your tongue and let the middle part of your tongue go down a little (the shape of your tongue will look slightly similar to "U"), then - rock-n'-roll (it meens vibrate the tip of your tongue against the top jaw, behind your top teeth). Even if you do a short R like in Korean or Japanese - you will be understood. But you goal is to make it longer than Japanese R and still not too long, otherwise it will sound strange )))