I'm not sure if you ask about the pronunciation of the letter NAME, or how to pronounce that sound. Letter name: erre (rr has to sound loud). Pronunciation of /r/ and /rr/: I always advise you to think of a motorcycle. BRRRROM. The tongue should vibrate in your mouth creating a sound similar to a motorcycle. There are several videos on the internet. Try to use them to learn the sound. https://www.youtube.com/watch?v=t3cmqQV1Jt8

You have choices. Hire a teacher with training in phonetics. Find a youtube video that demonstrates the underlying phonetics. The usual phonetic description is that the simple Spanish r is made by a quick tap of the tongue against the alveolar ridge behind the teeth and the trilled r is made by an oscillation of the tongue tip as air is forced between the teeth and the tip of the tongue.