Mix the colors partly on the palette and partly on the canvas. Begin by filling the main areas of light and shade with the correct basic colors and then add paint to modify them. Gradually add shades of color and gently spread the paint where you want it to be smooth with little visible brushstrokes.
Apply a lot of paint and leave it where you want pronounced textures as in an impressionist painting or use a paint knife to create striking textures. The contrast between smooth textures and pronounced textures, with embossed textures of “impasto” type and other carefully painted smooth parts, will bring life to the picture. Vary the texture “alla prima” that you create. Mix the paint with a little oil if you want to make a thin layer and carefully remove the brush strokes for the paint to be smooth. As long as it is still wet, you can add oil or paint to make this layer thicker or more diluted. Be careful, when it starts to dry or crust, do not apply less oily paint on top.
You can always do it if you want a particularly ugly special effect. For example, if you are painting a zombie head, you could add a big oily pocket to your cheek, let it dry in the wrong way, then tear the surface of the greasy paint so that the paint crust hangs and the lump red-brown oily paint dries on contact with the air, perhaps flowing from the tear. You can turn almost any mistake into a special effect once you understand how the oil painting works.