Breath out slowly. Put your lips in the position of the vowel i: the mouth is broad but the vertical opening is small. While breathing out, close your throat, in such a way that you feel the pressure on your lungs waiting to breath out, if only your throat were to allow the air to pass. Now if you succeed to close your throat slowly, you will hear a funny noise just when the opening is about to close, as if you are choking. This noise is a good approximation of the Dutch g sound.