Another trick for practicing rolling Rs is to replace the r in a word with a d. This works best if the R follows another consonant. So when practicing to roll your R don't say "bread" but try saying "bdead". If you manage to make that bd fast enough it'll turn into a sort of tongue slap that's a single "roll". From there it's simply a matter of practice until you can roll as long as you like.

Thats how people have always explained making the rolling R sound for Spanish to me which is different from a rolling R sound in German. I was never able to make the Spanish r sound when I took it high school.

In the "guttural R" used in German the sound is made in the back of the throat without the need to press ones tongue against the roof of the mouth; just the base/root of the tongue against the uvular.

https://en.wikipedia.org/wiki/Guttural_R