Soft light is very flattering and recommended for most portraits. There are no sharp edges, no harsh shadows (if any), also no wincing or squinting. On the contrary, harsh light highlights unwanted details, like blemishes, and emphasizes disproportionate features, like a bigger nose, thus making faces look less attractive.
There are two main rules: the larger the light source, the softer the light; light is softer when the light source is closer to subject and harder when the source is further away. (I found a sentence on someone’s blog saying: The secret of all good lighting is to make the light source larger rather than smaller.)
This means you won’t get much use of the camera’s in-built flash and an off-camera light is needed to create a soft effect. Professionals in studios use softboxes or umbrellas but if you have an external flash, you can bounce it off a wall or the ceiling. A big window will do wonders – you can choose a north-facing room or if the light is too harsh, you can cover the window with a delicate curtain. If you are shooting outdoors, an overcast day is best. On a sunny day a reflector or a flash light will be needed to fill in shadows underneath the eyes. The secret of all good lighting is to make the light source larger rather than smaller.
These photos taken at noon on quite a murky February day in Edinburgh. One thing I need to add, though – I think this model would look good in any light!