The German R is theoretically very easy to pronounce, however, it is exotic for English speakers, and there’s nothing even remotely close in Chinese, so it is worth studying. I’ll try to give you some tips.

End of syllable (semivowel):
I agree with Cerza that at the end of a syllable, you should pronounce it as a semivowel “a” sound, a bit centralized (between Pinyin A and E) IPA: /ɐ/ — very short and weak. (Some speakers don’t do this after a short vowel, using a fricative R instead, but you absolutely don’t need to bother.)

Gern /geeɐn/, sicher /zɪçɐ/ (note: /ç/ is similar to Pinyin X)

Beginning of syllable (fricative / approximant):
To elaborate on what Cerza meant by “guttural,” the tongue position for R at the beginning of a syllable should be the same as for G or a Chinese H, but with lots of extra friction. At the same time, it’s important to vibrate your vocal chords. Try making an NG sound, and then sliding into a very strong H, while growling like an animal. Some speakers use the tip of the tongue instead (either a flap or a trill), but this is not any easier, and it’s considered a bit old-fashioned or “country,” except in Switzerland.

If you have access to youtoube (I just met one Chinese who uses "VPN" to access youtube, maybe there is a way though, I am not at all a computer expert, but maybe you can figure out how it works).

So you could use my videos to get used to the pronunciation, because listening actively is very important to understand but as well to repeat the sound yourself: https://www.youtube.com/channel/UC2XoO1HBizGfvZR01kK_VJQ/playlists
Just on top you can use them for example while making a walk or something.
Feel free to tell me if you need a special topic! I shall talk about!!
I think it's the best method, cause me myself I learned that way and found it much easier than I've ever learned before.

1) This letter must be said as " éa" sound when you find meanly in the words middle. Example: Lernen ( léanenn)- the last consonant must be well pronouciated/ said. 

2) in the beginning must be said as Strong "R" but not so much.  Example: Richtig ( Ri-sh-tí-sh) so Rishtish.

3) When in the Middle must be said very Strong and gutural sound. Example: Schreiben ( SHÍ-RRÁI-BÉNN), so : Shírráibénn.

4) When you find " ER" in words end must be said as letter ( á) . Example: Lerner. So in this word we find the first rule and this last one. The pronounce will be: ( Léaná).  Other example: Lehrer.: ( Leará). The letter "H" in this situation has the "r" sound. And as I explained: ER=Éá,  / ehr= Éá .

I hope I helped You to learn some!