This is second hand from my friends that hunt, but here goes:
You have to use a vertical bow for waterfowl, no crossbows (Page 18 of the DNR book confirms this). It is legal to shoot the bird on the ground or in the air, though in the air you best use a Flu-Flu for safety. Use broad-heads of course. There was some confusion over shot placement when on the ground. One suggested shooting the bird through the chest from the front, while another said to shoot from the side to pin its wings if the shot isn't not immediately fatal.
If I'm not mistaken you need both a state and fed tax stamp to shoot them.
I have done it. I have shot them on an old friend's pond. I did body shots. They would duck their heads when I went for a head shot. I took a fishing rod with me. Shoot goose, retrieve with large topwater lure. If the shot was a total pass through, I would then use the lure to retrieve the arrow. I used the same philosophy I use for turkey hunting. Pin the wing or damage enough so they can't fly away while they bleed out.
I have only hit one on the fly. The couple times i have done it,i wait for them to back pedal a couple feet off the ground.
It takes 2 people but a good way to practice is fill up a old soccer or volley ball with non expanding foam and roll it cross ways and shoot at it. once you get consistent on the ground, start throwing it across in the air. you can also use foam discs to throw up to simulate one in the air back pedaling. its an expensive way to hunt though, arrows get pricey if you loose alot....But one of the funnest ways too.