Properly nailing plywood down is more complicated than just whacking away at a nail with a hammer. Because plywood is often used in support situations, where it is part of the underlayment for floors and roofs, the nails must be sturdy and secure the plywood well, without creating too much stress inside the layers of the plywood. What goes on top of the plywood is also important as it can influence the type of nailhead.
Place the nails that you're adding to the edge of the plywood at least 1/2 inch from the edge. The exact distance will vary by nail size; for example, the Association of Bay Area Governments in California says a 6d or 2-inch nail needs 5/8 inch of space from the edge, while a 10d or 3-inch nail needs 7/8 inch on housing exteriors.
Space nails out properly. Placing them too far apart lessens their combined ability hold the plywood down. Placing them too close together can place undue stress on the wood. As the nail shaft goes into the plywood, it forces the wood fibers apart. Have too many stressed, separated areas of wood too close together and you end up cracking the wood.
Use nails with suitable shaft and head types for the job. A floor or roof underlayment that has other materials going on top of the plywood will require flat heads on the nails. Smooth nail shafts are not suitable for work where the plywood is not supposed to lift up; a rough, threaded shaft will help hold it down.