For the best results, and the most forgiving light, use a softbox or umbrella at least two feet in diameter. Today, there are many softbox options for speedlights. All you need is one flash, a softbox, a light stand and either a cord or wireless trigger to fire the flash. I use a Lastolite Ezybox II Switch which can change shape from rectangular to square, and is large enough to produce smooth soft light. Placing this light high and slightly off-center to my subject produces flattering light with soft shadows under the nose and chin.
My favorite light to use for this technique is the Elinchrom Octabank placed about three feet away from my subject. This light is over six feet in diameter, and produces the softest light I've ever seen. Part of the magic of this light is due to the flash head aiming back into the softbox, diffusing the light before it travels through the diffusion panel on the front. Shadows are almost nonexistent, and skin tones look luminous. This light is expensive, but worth it for those shooting commercially.
2 REMBRANDT LIGHTINGSay you only have one simple light and no big softbox. What kind of portrait can you create with a single edgy light? Try using Rembrandt lighting for a dramatic look.
Rembrandt lighting got its name after the famous Dutch painter whose natural studio lighting produced a unique style of illumination on his models. Rembrandt lighting typically has one side of the face illuminated, and the other side cast in shadow except for a small, triangle-shaped area at the eye. Once you know what this lighting looks like, you will recognize it everywhere from magazines to movie posters to fine art prints.
Rembrandt lighting is easy to set up. Have your subject sit in front of a nice background, and place your light almost directly to their side aimed down on their face. Take a test shot and see where the shadows fall. By slightly moving your flash, you will get the perfect angle, so your light hits one side of the face, and just a small patch of light hits the opposite side of the face. Ideally, this small patch of light illuminates the eye.
You can use a bare flash head to create this kind of lighting, or use a larger softer source if you prefer. My favorite style of Rembrandt lighting uses a black background and a beauty dish for the light. The beauty dish softens the light, but it's still edgy. The subject just floats on the black background, a very moody shot!