2 processes I've seen/used. The one that appears to work the best is using the suction side of the pump to start the hole. As soon as you've pumped about 2' out you set the pole, then place the suction hose directly beside the pole. This will contnue to excavate the hole. You need to put a some weight on the pole (I've done it with a compact tractors bucket). When you set the pole to the desired depth you can allow it to fill back in (it will) or put a couple a bags of Sackrete around the pole (set's up pretty well under water).
This has worked well in a soft bottom. I've seen pole's set up to 6' deep in the bottom.