My advice - do not roll the Russian 'R' at all! Just say
'Betty bought a bit of better butter'.
it should sound american - something like (I can't attach files so you have to figure out the pronunciation yourself. - beddy bad a bid av bedder budder) the 'd' sound is close the Russian 'R'. When I first heard an American say 'Get out of here' (gedaudav...) I thought: 'well, he can say a perfect russian r!'.
maybe you ought to put your tongue tip a bit more backward in your mouth. Any questions? Ask me. And, btw, I don't care for my spelling, grammar or whatsoever