Just rinse it, cut it into slices and attach it to something in the tank with a rubber band/fishing line or weight. You can blanch them as well so they sink to the bottom. I actually cook them until they sink for my crays and put it in fresh for my pleco(who has yet to eat any of it).
I either boil the zucchini or freeze it to make it softer and both will help it sink to the bottom. I prefer to boil them cause they get real soft and they sink str8 2 the bottom. My Apple snails, crayfish, plecos, and other fish love these.