I put the tip of my tongue to the roof of my mouth, towards my teeth. I then breath out through my mouth while making the r sound and my tongue will vibrate back and forth producing the sound of a rolled r. I can't really explain it any other way.
Edit:
Clarification, you can't just hold your tongue to the roof of your mouth as it will just say there. As Ktulu said, you need to keep it loose so it can vibrate back and forth.