To realistically paint cat fur, you have to think and paint in layers. Slowly building up layers of paint and shadows will add depth and the illusion of softness to the cat fur you are painting. By blocking or painting in the colors and shadows first, you will give your finished cat painting the appearance of realistic cat fur. Practice new painting techniques on scraps of paper until you feel comfortable, and always use high-quality artist brushes.
Sketch the cat you are painting onto a canvas or piece of watercolor paper. Only sketch in large details (such as eyes, nose etc) and loosely sketch in where the colors change and become shadows and highlights.
Block or paint the first layer of color onto the sketch. Start with the darkest colors first as this will establish your shadows and depth. Let each layer dry completely before adding the next layer of color.
Paint in the background and let the painting dry completely. Once you have painted all the layers of shadows, highlights and the background, you can proceed to painting in the cat fur.
Thin your cat fur paint color to the consistency of ink using water or paint extender.
Push the 10.0 liner brush into the thinned paint and twirl the brush as you pull it back out. This will keep the liner brush at a point.
Paint in fine cat hair lines with the loaded 10.0 liner brush. Always paint in the direction the hair actually grows. Start with the darker colors first and pain the highlights last. You do no have to paint in each and every hair. Just painting in several fine cat hairs in colors ranging from dark to light will give the illusion of much more fur.
Clean the 10.0 liner brush frequently while painting to avoid clogs.
Using paint extender instead of water will enable you to easily wipe off mistakes.
Use very quick long strokes when painting in the cat's whiskers.
Try turning the painting upside down when painting the cat fur. This will take the focus off the painting as a whole, and make your brain focus on the details.
Paint in final highlights and details such as the cat's whiskers last.