Soak the pistachio in warm water overnight (or at least 8 hours).
Strain off the water which was used to soak the nuts and discard.
Pour the nuts into a blender followed by the four cups of warm water and any spices that you wish to include.
Blitz the entire contents for one minute.
Carefully tip the liquid into a cheesecloth or nut bag and give it a good squeeze. Repeat this process if required until you have separated the pulp from the green hue milk.
Pour the pistachio milk into a jug and add the cardamom, vanilla and use accordingly
Notes
Soak the pistachios in warm water DO NOT use hot water. While it may seem ideal in terms of fast tracking the process, you will be destroying all that goodness.
Ideally you want the nuts to soak overnight so they soften naturally. There's no harm in soaking them for several hours instead but the nuts may not be soft enough.
Try to use a powerful blender such as a Vitamix or Blentec, this equipment is designed for pulsing/blending nuts/water.