The rolling r (and rr)

[ElDanés]

Yesterday I read in one of my Spanish grammars that the rolling r's and rr's are made by trilling your tongue on the back of the teeth (i.e. dental), for the r, once, and for the rr, two or three times. But normally are rolling r's and rr's said to be made by trilling your tongue on the alveolar ridge (thereof it's formal name, an alveolar trill). Now I am wondering which of them is the correct way of doing it, or if it differs from speaker to speaker, and dialect to dialect?
Speakers (also of the English language, and other languages as well) tend to pronounce L's differently as well, some hold their tongue on the back of the teeth - and others at the back of the alveolar ridge. There might be a difference with the pronunciation of r and rr, as in the example, as well.
I know that some speakers of Finnish make their rolling r's from far back in the mouth, possibly as a velar- or uvular consonant, while others do it in other ways. So, again, it's possibly the r and rr are pronounced differently.

I tried checking how I pronounce them, and I'm trilling on the alveolar ridge, and, even if I really try, I can't make my tongue trill on the back of my teeth.

Some clarification would be good!

¡Gracias!