Sometimes it's helpful to think of the single and double r as two separate letters with two distinct sounds. Mis zapatos estan llenos de tierra. My shoes are full of mud. La carta tiene tres párrafos. The letter has three paragraphs. It's easy to assume the Spanish word carta is the word for card. This is a false cognate.
There isn't any word in Spanish that starts with RR. However, the R at the start of a word is pronounced like a "strong" R or RR. For instance, the R in "ropa" (clothes) is pronounced like the R.
Gone are the days when it was next to impossible to learn non-local languages.
Each pair is considered a single letter, and each has a singular sound. All the Spanish rr's have a very pronounced rolling sound.
More than 101 words in Spanish with double r (rr)
Children do this a lot when playing by themselves. The way you say those s sounds is exactly how ll is pronounced in Argentina and Uruguay. You can compare the sound you words with double r spanish, to the words with double r spanish they make, and see more as needed until here sounds the same.