I'm not 100% there yet - but the closest i've come is following these instructions
1) make the noise zz - as if immitating a housefly.
2) whilst still making this sound, move your tongue to the position it's in when you say L
3) form the mouth shape of an R
4) move your tongue very slightly off your palate
Please let me know if you have success with this method - it would be nice to know i'm on the right track.
It seems to be easier to say if you prefix it with a "P" - as in words like price, produce, price etc.