Calling High-Key photography a technique maybe somewhat far fetched, I like to think of it as an extension to “Expose to the right” (ETTR) with the intent of gaining a high motive contrast on a white backdrop. High key is basically brightening the mid-tones and shadows. high key photography is not just overexposing the shot. It is all about the exposure and the light. If you do not read the scene correctly, and try to shoot a scene where mid-tones and dark tones are dominating the scene, you will end up with an over exposed scene without gaining the high key image effect.
High-key photography in natural light
Perhaps the most essential to get right when shooting high-key, is the background. You want to find a uniform background, without any deep shady areas. A white background or one with more bright tones makes it easier to get the shoot right.
Once you have found a suitable background, then overexpose it by up to two stops carefully avoiding to blow out any part of your motive. Do also mind your shutter speed, if the shutter speed is very slow then a tripod is recommendable.