If you don't have a high speed blender like a vitamix, I suggest soaking your cashews in water for at least two hours beforehand. You can also do this morning of or the night before. This helps them get soft to blend easier. If you're in a time crunch, you can boil the cashews on a stove top. Fill a pot of water so that it's covering the cashews then bring to a boil, reduce and let simmer for about 10 minutes. Since I have a high-speed blender I don't do either one of these things.
Add all the ingredients to the blender and blend until smooth and creamy. You might need to scrape down the walls of the blender. If it's too thick, add a little more water. If it's too thin, let it sit in the fridge for a couple hours. The longer it sits in the fridge the thicker it'll get. If you're in a pinch and want it thicker just add more cashews. Always taste before you serve so you can adjust any of the seasonings, like adding more salt or lemon juice.