Setup is a breeze, taking just a few seconds as long as you're in a well-lit room.
During initial setup, you'll be prompted to set up face unlock. If you opted to skip past this step, you can find the setting in Settings > Security & Privacy > Face Unlock > Add Face Data.
After adding a PIN or a password as a backup security measure, scan your face by holding your phone roughly 4 to 8 inches (about 10 to 20 centimeters) in front of your face. Try to match the blue outline with your face, and wait for the blue progress bar to wrap around the circle.
When the phone is lined up properly with your face, the entire process takes just a couple of seconds. If it's having a hard time finding your face, you'll see a warning underneath the circle. Move the phone closer or farther away until it begins scanning again.
After the face unlock feature has learned your face, each time you wake the phone it will try to scan your face and unlock the phone -- and it's impressively fast at doing so.
As the setup process points out, face unlock isn't as secure as a PIN or fingerprint unlock. Apple's iPhone X True Depth camera uses nearly 30,000 reference points when trying to identify a face, while the OnePlus 5T uses 100 identifiers according to a post on the company's forum. To its credit, OnePlus also uses other software tricks to differentiate between a photo and a live person, but it's not perfect.