High key photography is a technique, which works well with young women, children, flowers and other beautiful subjects although it can of course be used to effect in other situations. The idea is to have the image lighter than normal and not to have any deep shadows BUT do not confuse this with not having full blacks in the picture. Sometimes, perhaps most times, a full black or almost full black is needed to add weight, contrast drama and compositional balance; occasionally a high key image may consist entirely of lighter tones. This is however less common.
The key to achieving satisfactory high key image is having suitable lighting which is set up and balanced to give even light and not cast deep shadows; my first attempt at the style failed for this reason. Most articles on the subject recommend three or more lights (flash or studio can be used) but it is possible with two diffused lights, as in my example flash guns shot through diffusers etc.) or even in soft natural light such as an overcast day would give.
The base for my example is my next door neighbour; I was taking pictures of her three year old daughter and grabbed the opportunity to snatch this single exposure of her with evenly powered flash left and right softened by diffusers. It is because this is a grab shot that the basic exposure is darker than it should be. It is always best to get the exposure as near right as possible in camera. The most important parts of post processing were not to lose detail by burning out the highlights, especially in the hair and to mask in a second version of the edited RAW file, at about one stop reduced exposure, to give more solid blacks in the area of the eye without which on this very blonde subject, the whole could seem washed out.