People have been painting for hundreds--if not thousands--of years. There really isn't a single method of painting that's correct or incorrect. Painting is a subjective experience. Artist express themselves in ways that are meaningful and personal. Because painting is such a personal experience, there aren't a lot of rules to follow. The techniques that we know of are optional at best. In short, painting can be done just about any way you like and no one can ever tell you that what you're doing is wrong. However, there are a few basic techniques that many artists use.
Beginning the Painting
Draw sketches before beginning. These studies form your decisions as you paint. Tape the studies to the easel somewhere that will be visible during the painting process. Many artists also use photographs to help develop an accurate representation of the subject. Even if you're working from a live subject, it's helpful to have pictures of the subject available at later times. Begin the painting by drawing a loose outline of your subject on the canvas. If you make a mistake, don't bother trying to erase it--usually pencil doesn't erase very well from a canvas. When you begin your painting, start with a light layer of paint quickly applied to the canvas. Add details later.
Switch to a smaller paintbrush to add details to a canvas. Thin the paint to blend details with the paint on the canvas. If you're painting with acrylics, thin the paint by mixing it with a few drops of water. If you're painting with oils, thin the paint by mixing it with turpentine. The smaller the details you're painting, the smaller the paintbrush you should use.
Enhancing the Realism
Look back and forth between the canvas and the photograph or live subject that you're painting. Constantly flick your eyes between the two images. Look for differences. Have the colors match. Avoid distorting the figure. Always look for areas of improvement. If you're working from a photograph, turn it and canvas upside down. This gives you a fresh perspective on the subject, allowing you to see the canvas in a new way. Make any necessary adjustments.
Differences in Application of Paint
Some artists apply thick layers of paint to the canvas, allowing the brush strokes to show. Other artists like the surface of the painting to be glossy and smooth, as if looking at a photograph. These two methods of applying paint create very different effects. While the textured application of paint allows the viewer to see the painting for what it really is--a painting--the smoother application of paint helps to create the illusion that the painting is a door into another world. Each application is valid, but the results are very different. If you prefer a highly realistic painting, use a smoother paint application. This means you will need to apply the paint to the canvas in thinner layers, always smoothing with the brush as you go.