For an opaque look, add one part water to one part paint powder. (For a stained look, you can add more water.)
It is easiest to make a paste of the powder with some of the water and stir until smooth, like making gravy. Then gradually add more water until you reach the desired consistency. Mix it thoroughly for 2-3 minutes, then let stand for 10-15 minutes so everything has a chance to disperse completely.
When mixing up larger amounts it may be easier to mix equal amounts of water and powder together using a whisk, electric milk frother or a blender.
Test it: if the brush drags, the paint is too thick, so you need to add more water; if the paint drips or if it is transparent, it is too thin, so you need to add more paint powder.