The 'f sound' /f/ is unvoiced (the vocal cords do not vibrate during its production), and is the counterpart to the voiced 'v sound' /v/.
To create the /f/, the jaw is held nearly closed. The upper backside of the bottom lip is pressed very lightly into the bottom of the top teeth. Air is pushed out the mouth between the top teeth and the upper backside of the bottom lip. This sound is to be a continuous consonant, meaning that it should be capable of being produced for a few seconds with even and smooth pronunciation for the entire duration.
The lips are kept mostly relaxed during the production of the /f/. A common error English language learners make is overproducing this sound by curling the bottom lip under the top teeth. This creates problems when linking other sounds to and from the /f/.