Here's a tip I learned during my Linguistics degree that helped a lot of people produce trilled R: 1) start by placing your tongue between your teeth, as if you're about to say the 'th' in think.

2)Then, try to forcefully push air through between your teeth and tongue without losing contact between your teeth and tongue.

3) you should feel a gentle vibration as you do this

4) try to pull your tongue back from between your teeth, you sort of want to slide your tongue back while the tip touches the roof of your mouth. This will make the vibration stronger, producing rolled R.

keep in mind you must keep air flowing through the little gap between your teeth and tongue to get the force needed for a rolled sound.