As a recent beginner I have a question about strumming - I have noticed that when playing my accoustic (a Yamaha F310) my pick is hitting the strings across the bottom of the fretboard, not across the sound hole.

I can start with the pick across the sound hole, but as I concentrate on chords etc, my strumming arm slides across the body of the guitar until the pick is once again across the bottom of the fretboard.

I would welcome any advice about how to get a better strumming movement. I have checked out the Fundamentals 1 videos again, but still can't get this sorted out.