First, start by positioning your subject far from the background. If you try shooting them up against a wall, for instance, you’ll create a nicely illuminated wall after you’ve implement the key light flash. So start with a background in the distance—a good 15 feet away, if possible.
Then, be sure to choose a vantage point without direct sunlight illuminating the face of your subject. Open shade (the shadow side of a structure, or under a tall canopy of trees) or even backlit by the sun should work. In fact, positioning the subject with the sun at their back will create a hair light that adds to the well-lit look. If all else fails, use a black flag or even a large silk diffuser soften the light falling on your subject.
If sunlight isn’t illuminating your subject’s face but you choose something like a normal daylight exposure for the camera settings (such as ISO 100, 1/160th at f/11), you’ll see what? An underexposed subject and a brightly exposed background. A silhouette, in effect. This is the perfect opportunity to introduce a flash exposure to balance with the ambient light on the background and illuminate the subject’s face.
You can certainly use TTL auto flash control, though I prefer to work in manual. At the exposure settings mentioned above, a hotshoe flash from six feet away may produce the correct exposure at, let’s say, 1/8th power. In the first example here, I held my unmodified flash at arm’s length above the camera in order to get it slightly off axis. After subtly tweaking the flash’s placement and output, you can balance the exposures perfectly—a daylight exposed background and a flash exposed subject.
What if you want to make that darkroom go darker? Easy. All you have to do is underexpose the background. That means you can adjust the shutter speed faster, though make sure not to go faster than the maximum sync speed of your camera (which is usually in the neighborhood of 1/200th). Adjusting the shutter speed won’t affect the strobe exposure. But because of the sync speed limitation, it’s not likely to make a big enough difference to produce a near-black background. Adjusting the aperture, however, will.
From f/11 and a 1/8th power flash producing a correct flash exposure, you can adjust the aperture to f/16, f/22 or even f/32 in order to underexpose the whole scene by one, two or three stops. Up the power of the flash from 1/8th to 1/4 and eventually half power, you’ll maintain a normal flash exposure on the subject while the background is underexposed significantly. That’s what I did here, but you can go even further if you’ve got the right tools—namely, a speedlight flash capable of high speed sync. With high-speed sync you can push your shutter speed beyond the 1/250th boundary and still create a correct flash exposure. The higher the shutter speed, the darker the ambient—and that means you can drop the background to pitch black if you want to.
You can also approach this technique another way, and that’s by working from the opposite direction. Start by creating a dark ambient exposure (say, 1/250th at f/22) and then simply add your flash until it creates an appropriate key light illumination. However you approach it, the trick to this “day for night” technique is overpowering sunlight with a strobe, and it’s a useful technique in all sorts of situations.