Using your old charcoal grill as a smoker for your back yard BBQ is Easy.
All you will need are 2 Aluminum Foil Pans, Charcoal Briquettes, Chimney and some Hard/Fruit Wood Chunks.  Start with a clean grill.  Load up your chimney with charcoal briquettes (make sure you have plenty left for adding later as necessary) and fire it up.  Let all of the coals get nice and hot. While the chimney is doing its thing, start setting up your grill for smoking.
Remove to top grilling rack and place one of the foil pans in the bottom (put this off to one side) and fill it with water.  Arrange 15-20 unlit briquettes along the open side of the pan.
Then, once the briquettes in the chimney are searing hot, put them over top the unlit coals and top with the wood chunks.  Place the grilling rack on the grill and place the second foil pan filled with water directly over the hot coals.  Place the meat to the side of the top water pan (the meat should be directly over top of the water pan that was placed on the bottom).
Place the lid on the grill and let it go. You want the temperature inside the grill to remain between 225-250 degrees.  If the temp starts to drop, you can add more unlit coals to the pile.
You can also add more wood chunks along the process to intensify the smoke flavor (be careful to not add too much too often, or your food will taste like an ash tray).  To gauge the inside temp you can use the built in thermometer in the grill (if it has one) or you can purchase a grill thermometer. The set up should look like this:

smoker setup