it's very simple, really. what you're essentially trying to do is blow out the window (and whatever is outside) behind the model by intentionally overexposing it. anytime you are shooting a backlit situation, and meter for the subject, the background is automatically going to be overexposed. the degree of overexposure depends on your exposure settings with either the aperture or shutter speed, or a combination of both.
while it is easier to do it if you have direct sunlight streaming into the window onto the model, it is also possible to accomplish this even if it is cloudy outside, or if the window doesn't receive direct sunlight. as long as the outside of the window is brighter than the inside, it is possible. and if you're shooting digital, it's that much easier.