Duck snorkeling is just a surfing technique employed by many surfers to punch through heavy whitewater or a breaking wave using comparative ease. To do it right, it will take practice and time.

Difficulty hard because the wave is coming.

Your traction on the surfboard ought to be on a third of it's length from the nose.
Since you're pushing down with your arms, you are going to push down with knee. This will submerge the tail of the surfboard. Watch an experienced surfer from beach and you'll notice that while the knee is shoving the tail down, another leg is kicking up in the atmosphere, giving greater momentum to the knee that's pushing the tail down.
Chances are you should be fully underwater and the tide will be passing overhead. As the tide is departure, keep shoving down on the surfboard, however attempt to maintain yourself level to the plank.
The back force in your knee that pushed the tail down, will cause the nose to lift. Pull up now with your hands and you also should pop out at the rear of the tide.
Since you may observe, there are few steps required in learning to duck dive a surfboard. But, it is an art that has a great deal of practice to get the time correct. If you start your duck dive too soon, you may submerge and pop up until the tide has completely handed. If you initiate the duck dive too late, then the wave will hit until you're submerged. Additionally, it requires a lot of practice to find the technique just perfect. Pushing the nose down is usually not too hard, it is with all the knee to drive down the tail which gives most surfers understanding how to duck dive the problem. Just keep at it, clinic the duck dive on smaller days, and use the eskimo roll (also known as turning turtle) on multiple occasions until you become more optimistic using duck diving.

It needs to be stated that duck diving is truly a move that is completed most useful with shortboards. Duck diving might be achieved on a funboard (mini mal) or perhaps a longboard but it takes much more push to have the nose underwater. When I surf with a long board, I decide to show turtle. I can't get enough downward push onto the surfboard to submerge the board satisfactorily beneath the drinking water. I end up losing a lot of ground as the whitewater pushes me back towards shore. I find for me, it is better to turn turtle and continue once the tide has passed.