The main difference between a smoker and a charcoal grill is that a smoker keeps the fire away from the food. A smoker contains the fire and keeps the temperature low enough to cook meats at temperatures around 200-250 degrees F. A grill is designed to cook hot and fast, but the average charcoal grill can do both. If you keep the fire small and to one side of the grill you can generate low temperatures that are the secret of real barbecue.
Fuel: To start off you need a charcoal grill, fuel (hardwood, charcoal, etc.), a way to light the fuel (charcoal chimney), a drip pan, a water pan and an oven safe thermometer. These pans can be simple aluminum pans you can get at most grocery stores. You will also need plenty of time and, of course, something to cook.
Building the Fire: Begin by removing the cooking grate from the grill and building a fire on one half of the grill. If there is any wind, it is important that the fire be on the windward side, which is the side the wind is blowing against. This is important because airflow is everything when it comes to smoking.
Assembling the Smoker: Next place the drip pan on the other side of the grill, directly opposite the fire. Now put the cooking grate back on the grill. When the coals are good and white you are ready to cook. Fill the water pan about 2/3 full of water and place it directly over the fire. This will add moisture into the air inside the grill. Place the meat over the drip pan, the thermometer next to the meat, and then put the lid on the grill.
Airflow: Now this is the important part. Depending on the kind of charcoal grill you have you need to set the vents so that the airflow comes in under the fire and out through a vent over the meat. This will draw the air through the fire, over the water pan and over the meat before it leaves the grill. What you need to do is adjust the vents to maintain the ideal smoking temperature of 200 to 250 degrees F.
Running the Smoker: While the food is cooking you need to keep the lid closed as much as possible, but you need to keep and eye on the fire and the temperature. You will need to add more burning coals to the fire periodically. Once you get the hang of it you shouldn't have any trouble keeping the temperature right. Now all you need to do is buy some meat and get smoking.