Squeeze out the acrylic paint into the cup. Start with a small amount. You may add more at a later time if needed. If you are mixing a composite color (two or more colors) put them into the cup together.
Swirl the paint in the cup. Use the brush or stick to blend it and spread it around.
Add a few tablespoons of water. You don't need a lot. The principle is to soften the paint and make it more liquid.
Mix well with the stick or brush. Uniform color and viscosity is the goal.
Pour the mixture into the latex paint. Stir vigorously. Keep mixing until every trace of the streaks of acrylic paint are gone. Ladle up paint from the bottom to see if there are unmixed traces. When the paint color is totally consistent, it's ready for application.