Hi!, this sound is a little bit difficult for people who doesn't speak spanish. Is something like this: you must to put your tonguetip on the palate, and make vibrate your tongue. The result of this vibration will be like the sound of a motor or the sound of a cat...XD You can practice with this: "Erre con erre, guitarra; erre con erre, carril; qué rápido corren las ruedas en el ferrocarril". Bye!

Here´s some audio help with both rr and r: http://www.studyspanish.com/pronunciation/oral-drills/pronunciation_rr.htm

it`s so easy only do the sound more large, that`s all