The /f/ sound is made through the mouth and it is Unvoiced which means that you don’t use your vocal chords to make the sound.
It is defined by position of your lips and teeth and it is a fricative, which is a sound that is produced by high pressure air flow between a narrow space in the mouth. In this case it is between the bottom lip and the teeth.
To produce the sound rest your top teeth on you bottom lip and force air out between your teeth.