The F sound is usually a labialdental fricative in English. So you are putting your bottom lip on your teeth and letting some air go by to make the F sound.

English has bilabial plosives where you touch both lips together and let air stop for a moment which makes the P or B sounds.

English doesn't have a bilabial fricative so you might be doing this in your dialect and it doesn't stand out to anyone because it doesn't otherwise have a phonetic meaning. But, interestingly, in other languages a bilabial fricative has distinct meaning from a labial dental fricative. I believe I've read that in Japanese the "F" in "Mount Fuji" is actually a bilabial fricative and not the normal F that English speakers use.