I get asked a lot about how to turn a SUP when surfing, and my answer is always "Move your feet more"
It doesn't matter if you are on an 8ft board or 14ft board, if you simply stand there in the centre of the board, you will not be able to get the most out of your turns.
I thought I would tape a gopro to the tail of my 10ft J-Hawk noserider to see how much footwork I actually do. This day was around head high on a sucky right hand reef break (with no one out)
The gopro ended up being under water more than I would have liked (will need to make up a higher mount) but you can still get the idea of how much my back foot moves from rail to rail during turns.