How? I used to hum along with guitar parts of songs back in the early/mid '70s...songs like "Jackie Blue", "Doctor my eyes", "Love Rollercoaster" (I love that opening riff), "Sara Smile" and a bunch more.
Then I'd take rubber bands and wrap them around the knobs of my bedroom dresser drawers and pluck them like they were strings.
Around when I was 11 or 12 my buddy Mark got a couple guitars and we used to play them and I started picking stuff up pretty quickly.
Then my parents got me a cheap acoustic...a Hondo II...and I played that thing for hours every day, picking up stuff from records and tapes and tv theme songs.
Why? 'Cause I thought is sounded cool...and I guess God had His reasons, too.