You can use the door to access the fire and add wood chunks to create smoke. One of the problems of these smokers is that the bottom rack (half the cooking area) is no good for smoking. The smoke is only held in the lid section so only the top portion of the smoker works effectively. In general it's big enough to smoke a whole brisket though. Once you have the food on the top rack of the smoker replace the lid and let it go. I suggest an oven thermometer for telling you the temperature. Place it inside the smoker near the food. Check it occasionally. Remember that every time you lift the lid you let out the smoke and a lot of the heat so leave it alone as much as you can.
Now after about 2 to 3 hours the coals in the coals pan will become caked with ashes and the airflow will simply stop. This then causes the temperature to drop and stops the smoking process. Now several people have tried to come up with modifications to the smoker design to eliminate this problem, but so far I really haven't seen an effective solution. You need to get the ash out of the pan to return to a good smoking environment.
Here's where it can get a little tricky but I came up with a solution I call the sift and its a little risky so be very careful. To clear the ashes from the pan remove the barrel section of the smoker to expose the coal pan. Start by standing up wind of the coal pan and with some very thick grill gloves lift the coal pan out of the base. These pans are generally aluminum and don't get too hot but you still need to be very careful. By carefully shaking the pan back and forth most of the ash will fall through the vent slits in the bottom. You can also use something to stir up the coals and get some of the ash to fall through that way.
Once you have the ash cleared out of the coal pan and into the base unit dump the ash out of the base and put the coal pan back in. The ash will contain hot sparks and needs to be put someplace where it can't cause a fire. Now you are ready to add more coals to the pan. The coals you add to the pan need to be burning and ready to go. A charcoal chimney works great here. With the fire restored you can put the barrel section back in place and resume smoking.
I want to emphasis that this procedure is dangerous. Don't do it with shorts on and wear eye protection. The fine ash is murder on the eyes and hot coals are just murder. Make sure that if you elect to do this trick that you do it away from anything flammable and that you have quick and easy access to water.
A few further tips to go by: Don't trust the thermometer built into the lid. I've used a couple of these smokers and each one reads differently at the same temperature. You want to reach a good temperature of about 200 degrees. I use an oven thermometer because it's more accurate. If the temperature gets too high thrown some wet wood chips on the fire. This will lower the temperature a little and build up some smoke. If the temperature gets too low open the little door on the front of the smoker to let additional air in to build up the fire. I recommend starting small and working your way up as you gain experience and get to know your smoker.