The first question you are going to ask is where to get these leaves? Bottled grape leaves can be found in most grocery stores. Corn husks can typically be found in the Mexican food section of your store. Banana leaves, on the other hand, usually have to be bought in specialty food stores, particularly those catering to Asian and Caribbean markets. Banana leaves should be relatively fresh. Always make sure you wash and dry fresh leaves before you use them.
As far as cooking with leaves, it is very similar to cooking foods wrapped in foil, except that you won't get an airtight seal. However, because leaves usually have their own moisture, this isn't a problem. Always work with leaves that are large enough to fit your needs. You can cut the food to fit and simply cook more packets. Fish can be prepared whole or in pieces depending on your needs.
To wrap fish in leaves, start with a clean leaf, brush lightly with oil to prevent sticking and wrap firmly but not too tight. Leaves can tear so you don't want too much tension. Experts can typically wrap up a fish fillet in such a way as to not need anything to secure it with, but you can use kitchen twine or toothpicks to secure your leaves. Threading a toothpick or skewer through the package will help it hold up while it's being flipped on the grill.