So far rolling the R (in words like hacer, abrir, perro, pero) works quite well for me. But I have a problem when the R is followed by the letter L, like in hacerlo. It works a bit better when I widen my mouth and say the word very slowly, but maybe you have any other hints for me. Where to put my tongue, how to open my mouth. Thanks!
The words that have an 'r' behind any consonant sound kind of strong, similar to the double 'r' like in the word "perro". You could try pronouncing first the verb "hacer" and then pronounce "lo". After that you slightly decrease the space between the pronunciation of them.
I hope this was helpful.