Welcome to the Workshop community @Steve3. Many thanks for your question.
Powder pigments can be mixed directly into waterbased paint. To aid the pigments in dissolving uniformly you can place them in a cup and add teaspoons of water until you get a runny consistency. Pour the contents of the cup into the can of paint whilst stirring it. Continue to stir the paint and pigment mixture until a uniform consistency has been achieved.
What type of project are you using the powder pigment paint on? I'm sure the community would be interested in seeing.
Please let me know if you have further questions or need any assistance.