From what I’ve learned and also hear, it’s like a soft R sound, not a hard R. It’s a bit similar to how you would pronounce R in Spanish. It’s more like an R-L sound. Like, try saying the R with the tip of your tongue behind the back of your upper-front teeth.
Having rhotacism (struggling with the American R), I actually fell into it. You'll have to focus on your tongue a bit to learn it, but I'll reverse the process I had to do in speech therapy for you.
Bring your tongue to where it sits when you say a hard R. Then bring it to where it sits when you make an L sound. Switch between them, focusing on the similar tongue position and how the two are just back vs front of the mouth and tip vs back of the tongue. The Japanese R is literally inbetween. Held without a corresponding vowel sound, it'll sound a bit like gagging.
Once you find that in-between spot with your tongue, you'll want to focus on sort of tapping your tongue to that spot, then drop it again. Start with a vowel sound, tap your tongue to that spot, and drop back down into a vowel sound. Swap out the vowels occasionally. This might sound like "ura, uri, uru, ure, uro". Swap out the first vowel to get used to transitions and build the muscle memory. Once you have that down, cut the first vowel short and don't vocalize until the tongue-tap to the roof of your mouth. And ta-da, you have a Japanese R.
It's also virtually the same sound as the flapped T that is heard in American pronunciation of words like better or atom. Easily confused with D by English speakers. Which explanation works the best for you will depend on which dialect of English and what other languages you speak.
