This is one of the major areas where romaji-users are destined to fail forever.
Forget the letter R even exists (because the letter R or its sound doesn't exist in Japanese).
Found a good video for a start.
Note that the lady gives you the bad examples between 0:22 - 0:30.
Place the tip of your tongue against the back of your uper front teeth.
(This is further up front than what you do when producing the English L sound.)
Release your tongue instantly after making a flap sound.
All you English speakers, feel free to correct my English just above if something doesn't make sense.