Need help to pronounce "rr" in spanish?

i am taking spanish class this summer. man, the rr sound is so hard to pronounce. i cant vibrate my tongue like spanish speaker. i cant find any video or picture that shows how to place my tongue. du vibrate the tip of your tongue, the middle part of your tongue, or the rear part of your tongue?
Update: yea i can say the single r, like in english
Update 2: yea but how to keep the tongue vibrate so long? like rrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr........
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