Items you will need:
Trailer
Used 250 gal propane tank
and old boiler tank with rivets
flat strap 1/4" x 2" (to go around door opening)
2-4x8x1/4 plate steel (to make fire box)
5' of 4" pipe (for smoke stacks)
4x8 sheet of expanded metal (for racks in side smoker)
a lot of grinding wheel
a lot of welding supplies (i.e. oxy/acet tourch, wire welder, arc welder, welding rod/wire)
turned mesquite handles
an umbrella
BEER
MEAT
and wood

Step 1: Find a Trailer

Buy a trailer:
Check local auctions or craigslist for good deals, I found this trailer at an auction and paid $20 for it. However you get what you pay for, I had to haul it back on a flat bed trailer due to fact the original rear diff was seized. Notice the fender damage you will see I repaired it a piece of sheet metal that I tack welded in place and used auto body hammers to form the curves. Then I applied BONDO to hide major blemishes.
I tore the old metal flooring out and replaced with 1/4" diamond plate (you can see that in the next step).

Step 2: Mount the Tanks

I forgot to take before pictures (I didn't think I would need any), but to start off I took out the old floor from the trailer and replaced it 1/4" diamond plate that I recycled from an old catwalk.
Then I cut the propane tank to approx. 5 ft long, since both tanks are 24" diameter I cut a saddle joint into the propane tank and mated it with the vertical boiler tank. I finally welded the vertical (riveted boiler tank) to my horizontal (old propane tank) prior to setting in the trailer.
Once moved to the trailer I welded braces from the horizontal tank to the diamond plate flooring. Likewise I welded around the bottom of the vertical tank to secure it to the floor. I then installed baffles inside the horizontal tank to control heat flow to the vertical tank.
I also fabricated the fire box located next to the horizontal tank out 1/4" plate steel.
Once the tanks were welding in place I cut the doors out and placed 2" flat strap around the doors to help seal the smoke in.

Step 3: Dress the Trailer, Mount Accessories, and Paint.

Here I covered the front end with left over diamond plate, installed a propane burner, and made the burner controls from an old header. I fixed the messed up fender, and primed and painted the smoker.

I mounted a door latch on the vertical door which compress the door shut to create a tighter seal.

Step 4: Build Awing

Finally I built a collapsible awing out of square tubing and corrugated metal roofing. I next installed thermometers
I built the horizontal racks from expanded metal and angle iron. For the vertical tank I purchased 4 22.5" round grates similar to the ones you can find for a weber grill. To mount those I simple welded pegs inside the tank and the racks just rest on them.

Step 5: Finally Use It....

Its designed to be one big smoker, or I can have two smokers. I installed baffles on the inside of the horizontal tank that can be shut off to make two separate smokers. So that I can cook at a higher temp and use hickory wood for chickens in the vertical tank and cook with a lower temp and misquite wood for my briskets in the horizontal tank.

So the final steps are to:
Season the meat start a fire and drink some beer.....