Two ways to do this.
One is a dry grind, just plain cashews with some sugar or salt (depending what you are using it for), not a lot just 1 tsp. Throw it in a food processor and grind. You need some abrasion to get a find grind and salt or sugar provides that. If you are using this for a sweet dish you can add more sugar. You can always stir in milk or water to ensure you get the paste of the consistency you desire.
Another way is to soak the cashews in some plain milk overnight. In the morning process them until smooth.