This sound is used in words like raam (window), maar (but) and varen (sail).
In general the Dutch letter r is a relatively difficult sound. For Chinese people there is another factor which plays a part: the r is sometimes pronounced as an l. In order to pronounce the r correctly it is important to understand the (physical) differences between the l and r.
Start saying the English sound 'l' (as in 'leg'). This sound is similar to the Dutch l. You will notice that the tip of your tongue is positioned behind the upper front teeth and touches (part of) the palate. When pronouncing the r, the tongue should not touch those parts but is positioned lower in your mouth, (almost) touching your lower teeth. Your mouth is slightly open. Then you need to do something with your throat. This can best be practised when you take a sip of water. Do not swallow, but gargle. This may require some practice. Afterwards, do the same thing without water.
Now you should be able to pronounce the sound r.