Hi. Interesting to think about the French rolled R compared with the Welsh. They’re quite different - the French involves raising the rear of the tongue whereas the Welsh is all about the tip of the tongue against the hard palate (behind the top teeth).
My tip would be to start by saying ‘ana-ana-ana’ repeatedly (or ‘yn y’ in Welsh if you like) and notice where the tongue touches as it goes up and down. You could think of this as a ‘lazy’ phrase - not much effort is required to make the sound.
Then to change it to ‘ara-ara-ara’ with the rolled R (or ‘ar y’ yn Gymraeg), you keep exactly the same physical motion of the tongue going, but you add in a bit more effort - specifically a push of breath shooting past the tongue and encouraging it to vibrate slightly against the hard palate, in the same spot it was tapping in the ‘ana’ phrase. If you place your palm in front of your mouth you should feel puffs of breath which aren’t there with ‘ana’.
If you keep the breath going you can vibrate the tongue continually, making a ‘RRRRRR’ drone sound like a generator. In normal speech, the R would last for just 1-3 or these vibrations.
Hope this helps!