Pick the right ginger
Do not get baby gingers or young gingers because it is the juice from mature or old gingers containing a starchy content that is good for curding the milk. When you crash an old ginger, you shall see beards inside its flesh

Stir well the ginger juice before hitting the milk
If the ginger juice is rest for a while, the starchy material just mentioned will settle to the bottom. That is why we need to stir the ginger juice before pouring in the milk.

Besides, it is better to pour the milk about 10 cm above the ginger juice. Perhaps doing such is to help mix well the two ingredients because you should NOT stir them after ‘milk hits ginger’.