How to roll your ';rrr's';?

Can't roll my r's to save my life. My tongue is too short. Last year I had minor surgery, lingual frenectomy, to remove the excess skin under my tongue but I still can't roll my rr's or whistle with your two fingers. How to I practice?How to roll your ';rrr's';?
push them down a hillHow to roll your ';rrr's';?
You can do the R still by doing it guttural.