I help teach a hammer making class in my local blacksmith guild, and the hammers we make are basic rounding hammers from square stock. The tools we use are a punch to make the whole, a drift to stretch it to size(but not to final size until faces are forged), tongs to hold the hammer through the eye and a hammer. in our class we work in coal forges so we can heat only the hammer face we want to work on. If I were to do this in a gas forge I would want be sure to use a water quenching steel so I could cool as needed. If you are planning on a more "Brazeal"(for lack of a better term) style hammer with the fullers behind the faces then there are more tools needed as stated above.