sitting on your board
is perhaps the most embarrasing surf thing to practice. Imagine a smooth paddle out, totally nailing the duckdives, get out to the line up, sit up confidently and then WOAHH, your board pops out from between your legs into the air, you go backwards into the water and then you end up missing the set wave of the day.
Still, the good thing is that this is quite a quick move to get the hang of and before you know it, sitting on your board will be second nature.
Your legs do most of the work with your body doing the rest. Sinista gives you the key tips, the rest is practice - go out on a flat day for an hour or two and you will have it nailed.