Achieving that hammered look on copper sheet can be achieved with some simple tools available in almost any home and you wont need torches or ovens of any kind. First, acquire an appropriate size piece of copper sheet. The important thing to remember in this task is to get the correct thickness and temper (or hardness) of copper sheet. Unless you really want a workout, I recommend a thickness of .021 inches. This thickness of material can also be called 24 Gauge or 16oz. When selecting the temper, request soft material. This temper copper sheet will accept the hammering marks more easily and will result in less sweat on your forehead. Also, soft copper sheet is more pliable and can formed by hand, if you need to bend it.
Next, you will need a backing board for the copper sheet. If you have a piece of plywood, that would be great. Normally, soft copper sheet comes rolled in a coil. Unroll the copper sheet and secure it to the plywood with a nail in each corner. Try to get the sheet as flat as possible. Now, place a towel on top of the copper sheet.
Now, a hundred years ago, your typical coppersmith would have a dozen hammers in his rack to provide various functions and finishes on the copper sheet. However, we will use just one, the ball peen hammer. The rounded end of this hammer will provide the indentations we most closely associate with that hammered look on copper sheet.
When hammering the copper sheet, let the weight of the hammer do the work. You really dont have to hit the copper sheet very hard to make an indentation. Many softer hits close together will look better than a few big dents. Keep in mind that many different finishes can be achieved with this process. Also, if your backing material has a texture, hammering on it will emboss that texture on the copper sheet. One of the more interesting looks can be achieved by hammering the copper sheet on concrete. The pocked and gritty surface resulting from this backing gives the copper sheet an aged and distressed look that is quite appealing. Clearly, the sky is the limit when it comes to the variety of finishes you can hammer into this durable material.
An important word about safety, always wear eye protection when you are hammering copper sheet and gloves do help when handling the sharp edges of the copper sheet metal.
Good luck on your project of hammering copper sheet.