Liquid acrylic gives you more flexibility in application than acrylics directly out of the tube or jar, which have a heavier composition. Use liquid acrylic especially for pouring techniques, or for covering large areas of your painting. Liquid acrylics can be purchased commercially and have high pigmentation, but you can do your own at home that are good for most uses.
Decide what colour you would like to use and mix it on your palette using the palette knife. To make 250 ml (8 oz) of fluid paint, mix about 60 gr (2 oz) of acrylic paint. Get as close as you can to the desired colour, but if you need to make a change later, you can. Make sure there is no "broken" colour, in other words, make sure that the colour of the mixture is uniform. It is worthwhile to make a little extra mixture and put it aside in case you need to make an adjustment later.
Put the paint mixture into the can or jar. Add 170gr of water and stir well until the paint is well dissolved and consistent. If you find that the liquid acrylic is too watery, add some of the acrylic medium to give it more body. If you don't have medium, you can add more of your paint mixture. A very watery mixture will act like a watercolour; thicker mixtures will be more opaque. Finding the balance for your particular usage is the key.
Make your final adjustments for colour, thickness and opacity. To alter the colour, mix paint on the palette and add it a little at a time, stirring well after each addition. It usually does not take a lot of paint to change the colour significantly. If you find the paint is too thin or thick, add either medium (or more of your original paint mixture) to thicken, or water to thin. If the paint isn't sufficiently opaque, add more of your original mixture; if it's too opaque, add small amounts of water and mix well before adding more.
This method of preparing paint is not recommended for use in airbrushes. If you are using an airbrush, check the instructions from the manufacturer regarding paint mixtures. Some airbrushes require specific paints, and using the wrong product can damage them.