How do I set the Nikon D90 timer mode for capturing self-portraits or group photos?

To set the timer mode on the Nikon D90 for self-portraits or group photos, press the "Menu" button, navigate to the shooting menu, select the timer mode option, choose the desired delay time (usually 10 seconds), and then press the shutter button to start the timer. Position yourself or your group in the frame and wait for the camera to capture the photo.