Daylight from a window provides a soft light, and high degree of contrast/modelling. Windows that face the sun are generally not very satisfactory because the light can vary in strength from moment to moment. Direct sunlight will also cast strong shadows from window frames, leaded panes, etc.
The size of the window affects the quality of the light. A small widow can act like a soft spotlight, while a large window provides a more diffused light like a floodlight. Similarly, the proximity of the subject to the widow will affect how they are illuminated. The closer they are, the softer and more even the lighting will be.
Your aim should be to place the subject so that the light falls on their front, and not behind them. The subject should be facing roughly parallel to the window. They can turn to look through the window, but should not be looking directly out of the window, nor into the room and away from the light.
The bottom of a large (e.g. floor length) window can easily be masked-off (newspaper and tape will do) so that the light falls from above at an angle of between 30 and 45 (see the basic principles of lighting).
The light from a single widow will often produce too much contrast, and a “fill-in” light is usually required.
Fill-in illumination should not be brigher than the main light source.
A white reflector is often the simplest way to provide fill-in illumination. A large piece of white card works well, and is easy enough to obtain if you keep your eyes open for such things (or you can buy a sheet of white mount board). The aim is to use this to reflect or bounce light from the window back on to the subject. You should be able to clearly see the effect of the reflected light as the reflector is repositioned.
The closer the reflector is to the subject, the stronger the reflected light will be, but the intensity of the bounced light will always be less than that of the main light source.
Another option is to use an on-camera flash. This is a little more tricky to get right as the level of fill-in achieved can only be observed once the picture has been taken. If the level of illumination provided by the flash is greater than that from the window, it then becomes the main light source, and modelling is lost. The strength of a flash can be reduced by sticking layers of tissue over it.
Normally, the position of the fill-in light will be on or near to the axis of the camera. This makes an on-camera flash ideal when used as a fill-in. The position of a card reflector should similarly be such that it bounces light on to the subject along the axis of the camera.
Don’t expect to get the lighting right first time. Some experimentation will be necessary, but the results that can be obtained should make the effort worthwhile.
If there are two windows, lighting becomes much easier, because there will be a greater amount of ambient light, but you may still need to use a fill-in, and mask-off the main window.
If the two windows are on adjacent walls, lighting can be even easier, since one window can function as the main light source, and the other as the fill-in. The aim is to place the subject nearer to one window than the other, and adjust their position as necessary to achieve the required balance between the two light sources.
Backgrounds will always come out much darker when using the available light from a window. Due to the significant fall-off in the level of illumination, the further away the background is, the darker it will appear. The background will also reveal whether the light from an on-camera fill-in flash is too strong. You should not be able to see a shadows cast by the flash. If you can, the flash is probably too bright.
When taking portrait shots outdoors, we have far less direct control over the available light. However the principle of fill-in illumination still applies. When daylight cast heavy shadows, these can be reduced by use of a reflective surface to bounce light, or an on-camera flash.
Outdoors, the use of flash is a better option, as the main light source will usually be far brighter than the flash light, and it can be extremely impractical to carry a large sheet of mount board with you.
Footnote: There are circumstances when these “rules” may be broken, but the intention of this article is to convey simple guidelines applicable to most situations.