Find a quiet space to tune in. Since digital tuners use a small microphone in their audio inputs to detect sound, you will need to be in an environment that has little ambient noise. If there are other instruments playing or people talking, it will give your tuner a false readout.
Set your tuner to tune to the note "F." Digital tuner/metronomes have a chromatic adjustment that allows you to pick which note you are tuning to. Different instruments use different notes to tune with. According to Musicbasics.com, since the French horn plays in the key of F, you set the tuner to F.
Play an F note on your French horn. As you sustain the note, watch the readout of your tuner/metronome. You want the needle that detects frequency to be in the center and the green light to light up. If you are sharp and the pitch is too high, the red light will light up and the arrow will point to the right. If you are flat and the pitch is too low, the blue light will light up and the arrow will point to the left.
Adjust the tuning slide on your horn accordingly. If your F note was flat, push your tuning slide in until it is in tune. If you were flat, pull your tuning slide out until you are in tune.