Silhouettes show the outlines of the face but still have it hidden.

So, how do I do that?

Backlighting is Key
The background must be significantly brighter than the foreground, so this is best to do at sunrise or sunset, but it is still possible at other times, but maybe a bit of editing to get the contrast higher so you can see the subject.

No Front Light
Turn off your camera flash. You want as little light hitting the front of your subject as possible.

Camera Settings
Shoot in Manual Mode: Set your camera to Spot Metering, and meter for the brightest background area. Use a small aperture (f/8 or highter) to get everything in focus, but do keep your ISO as low as possible.

Phone Settings
Open your camera app and tap on the brightest part of the background,  adjust the brightness using the slider with a sun icon and then drag the slider down to darken the image until your subject is completely black.

If all this fails
You can always edit to get your subject blacker, and your background brighter, looking forward to seeing your hidden face.

Have fun, if you don't want to do a silhouette, then maybe pareidolia is for you.