always use fresh moist paint from the tube. any of these ways works. if i need to mix a large amount of pain i squeeze out color from the tube.
1. put the amount of water that you guess you will need to cover the amount of paper you need to cover with the color you want in a palette flat, or palette well, or container, preferably a white-colored palette or container. now i add that much water again because i usually run out of paint before i complete a wash and then the edge dries while i try to mix more paint or i don't get the color exact again, 