This is hard to tell. With no videos, I do not know how to explain. Try singing, "eeeeeeee", then lowering pitch lower and lower. As you do this, move your chin to your chest. When the pitch of "eeee" is low, and you are looking down, the "eeee" may change to "errrrrrr".

Oh wait I just realized this is to help with high valyrian not english, I'm so sorry!

It is always a rolled or taped 'r' like in Spanish for example. For some people it's quite hard to do, also for me. I'm a German and I don't have this rolled r in my native language so I cannot make this sound. I really have to practice it a lot.
我希望我幫助了你。加油。

找到战友了，可以加我微信mrleftsir，我们一起学习呀