Practice Spanish speaking

I know it's hard pronouncing some sounds, for you it's the RR, for others it might be some Chinese sounds, or Arabic sounds: it can happen to anyone when trying to pronounce some unfamiliar sound.

So don't stress about it :)

Now, how do you pronounce an R ?

I'm going to try to explain the mechanics of it for you (for a simple R and then for a RR):

First, you put the tip of your tongue on a part that is called the alveolar :
The alveolar ridge - is the elevated area inside the mouth exactly behind your teeth (so your tongue does NOT touch your teeth)
Then you will push air in between the alveolar and the tongue in order to vibrate your tongue.

If you try switching from L to R you will see the difference in the fact that when pronouncing a normal L, you will touch your teeth.
To switch to an , you will put your tongue BEHIND them, so behind the spot you touch when saying L.
In doing so you will also notice that the tip of your tongue arches more upwards (some people describe this as moving or arching their tongue backwards).

To pronounce the RR, you will simply put more accent in pronouncing and vibrating(flapping if you will) your tongue.

How to do this accentuation:

You will produce a few flapping per second to pronounce the RR ( as opposed to R where you'd produce 2, 1, maybe even none), you will keep it for a longer time, and you will engage your mouth more to emphasize the sound, also the preparation for the sound and the release after it will be greater.

To understand that accent think of it this way:
The quality between the R and RR sounds is like the quality between the E and EE in :

m
and
Bungee (bungee jumping)

While writing this, I realize that I've been explaining to you how someone speaking a ROMANCE* (latin based, except French) language pronounces the R.
*In english the movement is the same, but you not vibrate the tongue, and you actually leave some tiny space in between the tongue and the alveolar.

Here are some words for practicing

in Spanish:
real - leal (real - loyal)
pelo - pero - perro (hair - but - dog)
calvo - caro - carro (bald - expensive - car/cart)

And in English:
right - light
pretty - plenty
pray - play

Some tricks that might help you on pronouncing the RR are trying to:
- imitate a cat purring
- imitate a car engine starting (motorcycle maybe?)
- make the brrr noise you do when you're cold (that's when your mouth is trembling so that helps with the vibration right there)