There are several reasons why plaster is a good material to incorporate into the paint you use in your artwork. It is cheap and available. You can obtain it at almost any hardware or art supply store. It mixes easily with paint. A few turns of a stir stick blends it in adequately. Finally, it's durable. You don't need to worry about it decaying over time. Be careful of a couple things, though. Too much plaster will weaken the paint. It can even cause the paint to crack. Also, before applying it to your canvas you might want to put the canvas over wood. A wood, composite board or similar panel, with canvas attached, fixes the surface area of the canvas. This cuts out canvas "sway". The sway can exacerbate cracking of heavy paint.
Mix your colour of choice. Use the palette or a container. Larger amounts mix more easily in containers.
Add plaster. Start with a tablespoon. Mix it fully. You shouldn't see any white plaster.
Evaluate the mixture. The plaster makes the paint stiffer and grainier. To make it stiffer (or grainier), so it will have more texture, add more plaster. Add only a tablespoon each time. If the mix is too stiff, add more paint. Don't exceed a 50-50 mix of plaster and paint.
Apply the paint. Put it on areas you want raised or areas where you want a heavy texture. Scoop it up with the spatula. Swipe it onto the canvas. Manipulate it with the spatula or palette knife. You can mix and add more plaster-paint mixture if you wish or use other colours.