Flat water jumping is very similar to ollieing a skateboard. First off wedge your feet in the straps. Next pump to get as much speed as you can. Make sure you are over the board and unhook with your back hand a bit further down the boom. The next step is described as a push pull pull pull. Push the tail of the board as well as getting a lower body position. Get ready to explode as you pull up with your front foot and up on the boom while sheeting in the sail! Then pull up your back foot to your butt and you have air off flatwater. Point your toes to get more air under the board as well.
First just try getting more and more enthusastic with pumping in the straps. You will find that the board will bounce around and even get a bit of air.
Do the same thing but time it starting with pushing the backfoot down to raise the nose when you are in the trough of the chop or wave.
Enjoy and good luck,