Smokers are actually basic cooking devices. They generate smoke and heat in controlled portions while providing the space and versatility needed to smoke a wide range of foods. The basic idea is to have two areas in the smoker. One area is where heat and smoke are created at the necessary temperatures to do the cooking while the other area holds the food in a temperature controlled, smoky environment.
The vents and connections between these spaces provide the airflow necessary to keep the heat and smoke traveling so that an excess of smoke doesn't build up that can make food bitter.
Airflow: If you take a look at this Double-Barreled Smoker you can get an idea of the basic principles. One chamber has the fire while the other holds the food. Connecting pipes bring smoke and heat into the food chamber and smokestacks vent smoke out of the cooking chamber. This is important because and excess build up of smoke will ruin your barbecue. What you can't see from this illustration is the air intake vents in the firebox. In a working smoker, convection draws in the air over the fire. The heated, smoky air rises into the cooking chamber and then vents through the stacks. This airflow is the most important part of a smoker.
Venting: You can achieve temperature control through adjustable vents which limit airflow or you can do it by using a controllable heat source.
This is the easiest way to smoke because you don’t have to deal with fire tending and vent adjustments. This also allows for a very simple smoker design. Take for instance this electric Trash Can Smoker. The design is very simple. An electric hot plate provides heat while wood chunks in a pan on the hot plate provide smoke.
The temperature control of the hot plate allows for direct heating of the cooking chamber. This can also be accomplished with a gas burner.
Portability: Now while you've got the basic principles in mind you first need to think about whether or not you want your smoker to be portable. By portable I mean do you want to be able to move it. Smokers can be permanent fixtures on your property or they can be set up and moved if necessary. For instance Dave Lineback's Wilber D. Hog Smoker is a beautiful brick unit with lots of versatility and plenty of smoking space, however, he can’t exactly take it with him.
Materials: Brick or stone smokers have a lot of advantage over other kinds of smokers. Brick holds heat so once one of these smokers is up to temperature it will hold that heat very well. This levels the heat spikes that can occur in many metal designs. With a permanent structure smoker you can also add in all the features you might want. Electricity and plumbing can be added to the unit giving you a lot more versatility to your outdoor cooking area.
Of course, just because you want to use bricks or blocks doesn’t mean you can’t make a temporary or non-permanent smoker. Dan Gill’s Rufus Memorial Smoker is a great hog cooker based on a simple design.
This classic unit is perfect for a temporary smoker if you are planning a big “pig pickin’” or even a luau. This goes to show that a smoker can be built out of almost anything you have on hand.
Now that I have you thinking about whether or not you want a permanent, fixed smoker you need to access the most important resource you have on hand. Your abilities. If you are up to welding and masonry then you probably have all the skills you need to make any kind of smoker. However, if you don’t feel you have the skills or perhaps access to the tools you should try looking at a different type of smoker. It’s best to start out with what you know and work your way up from there. Many of the people who have made impressive smokers have started out small and worked their way up. Nothing counts like experience.
In recent years, the hot item in the build it yourself smoker is the Drum Smoker, or Ugly Drum Smoker (UDS), though increasingly they are not very ugly. Made from a 55-gallon steel drum, these are easy to make, easy to use, and work very well. While there are a number of people making this style of smoker, there are also several making DIY kits that include everything except the drum. These kits tend to cost up to $200USD (again, without the drum), so this isn't exactly a discount proposition, especially when you consider that you can buy the Pit Barrel Cooker for around $300USD. The advantage to making your own drum smoker is the experience and that it can be customized however you wish.
Whatever material you choose to use it needs to be durable enough to withstand the temperatures. It needs to take the abuse over time and not contain parts that will break down or produce fumes. Stone, brick, and metal are all good choices. Beyond that, you could get into trouble. So no matter what type of smoker you choose to build, plan it out well, experiment as you go and have fun. That’s what it's all about anyway.