This is the best guide to making alveolar trills (like the Spanish “rr”) that I’ve come across, and believe me I’ve been trying to make them for over a decade.
The previous best advice I’d gotten about alveolar trills was, if you can make bilabial or uvular trills (which I can), you can carry the trill feeling forward or backward as applicable. Which worked for me, but is not terribly practical for doing inside each word.