Among oil painters, there seems to be a common misconception that glazing is some kind of mystical technique that only a few can master. The basic process is, however, very simple. Glazing is putting one layer of paint over another so that you can see the underlayer through the upper layer of paint. Glazing is a form of indirect painting, which just means that you are painting with more than one layer, allowing previous layers to dry before you add more paint on top.
Glazing can be used for a number of purposes. As I noted my post comparing the glazing methods of Italian and Netherlandish Renaissance painters, glazing can be used to create optical color mixtures (a blue glazed over a yellow makes a green) or to create modeling effects (thicker layers of transparent paint are darker, so you can adjust value by adjusting the thickness of the paint). Some artists glaze over a whole painting to unify the overall tone. Others will glaze specific parts of the painting. One method is to do an initial monotone underpainting (in shades of grey, for example) then apply color over it. This simplifies the process of painting by first tackling pure value, then working out color. Some modern portrait painters will do an initial painting of flesh in shades of green (they incorrectly call this a "verdaccio"). They then glaze with reds and oranges (complimentaries and near-complimentaries to green), providing the flesh tones with a sense of vitality that is difficult to achieve with direct painting. Glazing can also be useful for maintaining chroma in light colors. Mixing with a lot of white will seriously reduce the chroma of most colors, resulting in a look often described as "chalky." If you glaze the same color over white, however, you can achieve an optical effect that is high in value, with more chroma that you could get by mixing that color with white.
Because a glaze darkens what it covers (unless its a scumble—see below), it is best to do the underpainting lighter than the intended final effect. If you are going to glaze, it's important for the underpainting to have as smooth a surface as possible. That's because irregularities will trap excess amounts of paint in the glaze layer, creating wierd little spots of darker paint. So, before the paint dries, it's a good idea to go over it very lightly with a soft dry brush, looking for lumps and gently brushing them down. After the underpainting has dried thoroughly, you may want to wet sand to create as smooth a surface as possible.
In selecting paint colors to glaze with, it is useful to distinguish among opaque colors (like cadmium yellow), semi-transparent colors (like ultramarine blue), and transparent colors (like alizarin crimson). While any of these colors can be used for glazing, transparent and semi-transparent colors are darker when they are put on more thickly. Opaque colors can be used for glazing, but only when they are applied in a thin layer. A thick layer of an opaque color is not a glaze, because you can't see the underpainting through it.
Many oil painters think that the best way to glaze is to dilute the paint with an oil-resin medium to a watery or syrupy consistency (this is what a lot of art instruction manuals tell you to do). The paint becomes less opaque because the pigment particles are separated by a larger than normal amount of transparent vehicle. This type of glaze is called a dilution glaze. In my (deeply humble) opinion, it's the wrong way to glaze. It's bad technique for (at least) three reasons: (1) all of that extra resin and oil will darken and yellow over time, ruining the effect; (2) dilution glazes tend to create a sort of "tinted photograph" effect that doesn't have the solidity a painter is usually trying to depict; and (3) the documentation I've found on historical glazing techniques suggests that only small amounts of resin are detected in glazing layers in Renaissance Netherlandish paintings, which I consider to be the gold standard in glazing for both beauty and longevity.
A better method is called a reduction glaze. This approach involves adjusting the transparency of the paint by adjusting the thickness of the paint layer. While you can do a reduction glaze with nothing but pure oil paint, it helps to first lubricate the surface by applying a very thin layer of a slippery medium. My preferred glazing medium is a 50/50 mixture of black oil (linseed cooked with lead) and Venice turpentine (if you don't like to use substances containing lead, linseed oil will work almost as well). Studio Products also sells an excellent glazing medium. Put a drop of medium on the surface, rubbing it in with your fingers to spread it as far as possible. This way, you can cover a large area with just a few drops of medium. In addition to putting some on the surface, you can also put just a tiny bit of medium in your paint, but I don't usually find that necessary.
Mix up the color you want to glaze with. Apply it thickly and evenly to the desired area with a brush. It will look like a horribe mess at this stage. You will now reduce the thickness of the glaze to the desired opacity and value. Do this by dabbing with a soft brush, smearing with your fingers, rubbing with a cloth or sponge, or whatever works to adjust the glaze to achieve the desired effect. With a little practice, a reduction glaze is really pretty easy. You can get nice gradations in color and value by creating a gradation from thin to thick. Or you can create gradations from one color to another. Once you have the glaze spread to the right thickness, you can, if you like, paint into it with other colors. For example, you can apply light highlights into a wet glaze and then blend it in. If desired, you can let your glaze dry and then add one or more additional layers of glazing. For example, you can get really intense, chromatic darks by glazing with multiple layers of transparent paint.
When mixing colors for a glaze, it is sometimes helpful to add a small amount of white to your mixtures. This provides a greater sense of solidity. If you glaze with very light colors containing a lot of white, it is usually called a scumble. Titanium white, being very opaque, can be a bad choice for scumbling. Flake white and zinc white are much easier to create transparency effects with. A very white, hazy glaze is called a velatura ("veil"). A velatura can be a great way to depict transparent smoke, haze, or fog.