This sound is used in words like raam (window), rijden (drive) and waarom (why).
There are different ways of pronouncing the Dutch r. The so-called 'rolling r', a sound that is made somewhere at the back of your throat, is used at the start of a word, part of word (e.g. beroep) or after a consonant (e.g. praat, krijgt, etc.). At the end of a word the r can have different pronunciations: the 'rolling r' or a sound similar to the American English 'r' (as in 'far'). In this exercise the 'rolling r' is discussed as this is the most common sound and regarded as typically Dutch.
In order to be able to pronounce this sound correctly, start saying the American English sound 'r' (as in 'far'). Your mouth is slightly open. You should drop the back of your tongue a little bit, so that it is positioned lower in your mouth and more towards the back, (almost) touching your lower teeth. Then you need to do something with your throat. This is the most important part. This can best be practised when you take a sip of water. Do not swallow, but gargle. Your throat should now make a 'rolling' sound. This may require some practice. Afterwards, do the same thing without water.
Now you should be able to pronounce the sound r.