Most of what people learn about exposure trains them to avoid backlight – meter for the sky and the subject goes dark, meter for the subject and the sky blows out. Silhouette photography just picks a side on purpose.
I meter for the brightest part of the sky, a few stops down from where the subject would look correctly exposed, and let the subject fall to a clean, solid shape. A clean outline matters more than detail here, so I ask subjects to turn slightly to the side, keep some space between their arms and their body, and avoid overlapping limbs that would blur the silhouette into a blob.
Shooting through a slightly wider lens and getting low, closer to the horizon, exaggerates the sky and makes the silhouette read bigger against it. It is one of the few techniques where a plain, empty background is exactly what you want.