I purchased a welder last year and by law if you live in Texas and don't build a smoker, they will take it away from you. 

Google images was a great research source for ideas.  I saw lots of smokers and noticed that the better ones had some kind of baffle plate between the firebox and the main chamber.  I used a piece of 1/8 plate and it worked great!!  I also wanted to make sure that the firebox was not too small. 

I started with an old water heater, stripped the insulation and outer sheet metal, cut off the pipe fittings and exhaust.  I did not know that these tanks were convex on the bottom.  I cut the bottom off and flipped it over to gain a few more inches and the right shape.  In retrospect, this was a bad idea.  The metal that was around the bottom was not only a different diameter as the tank it was also much thinner metal.  I went back later and  added a reinforcing plate.  If I were to do it over again, I would still flip it but would have not used the thinner metal that was the below the base of the tank.

It took quite a while to get the legs set right.  These hit the ground square and it doesn't wobble. The bottom shelf helps keep them in place.

The firebox is 2/3 of and old gas tank off a semi.  It had been rusting in a field for several years so no issues with fumes.

The door for the firebox is my favorite part of the smoker.  I really like how it came out.  I got the vent from http://www.kck.com/ and picked up the handles at Elliott's (www.elliottshardware.com)  I made the hinges from 1/8 plate and they work great.

The lid is also made to be removable.  It will slide off to the right.  These hinges were just 3/8 rod and some pipe.

I could have used a roller to help shape the trim for the lid.  This is one part I was not happy with.  I tried sealing it with chimney rope seal but the adhesive did not last.  There was some warping when I attached the handle to the lid, which did not help.

Once I got the main shape done I thought it would be a good idea to burn off anything that would burn off before I painted it.  This was also a great motivator to see it smoking.

I searched for replacement grates and couldn't find anything that I liked.  So I made my own out of 5/16 rod.  Had to weld a jig to my welding table for this.  They are very sturdy.

After that, I added the front shelf.  The paint was supposed to be cured for 1 hour at 350 degrees, I did my best but  the paint on the firebox came off the first time I used it.  I've since put a second coat on it  and it has stayed.

One of the other things I noticed about some of the better smokers I saw online was that the smokestack was attached in the middle of the smoking chamber.  I did this and it helps keep the smoke filling the whole chamber.  Made the smokestack from the exhaust pipe that originally ran down the center of the water heater. I added the cap to keep ran out of there.

One of the unique features I added was a rod near the top to hang meat.  the grill grates could be pulled out and you could probably fit 10 chickens in there.  It works great for turkey legs....mmmmm turkey legs.

On it's maiden voyage, I smoked 2 chickens, a pork butt, 6 pounds of sausage, turkey legs and a whole pork loin.  Everything came out great.  I was most impressed by the flavor it added to the sausage. It was unbelievable! 

As for using the smoker, it was a breeze.  It is almost impossible to get it too hot.  The temperature stays right in the 225-250 range.
The last time I used it I shut the door when it was done and it stayed hot for several hours.  The next day when I went to clean it out, some 18 hours later, it still had hot coals.