We just made a longtail cargo bicycle from a second-hand Ukrainian mountain bike. It was made in Haiti with a minimum amount of tools and measuring, but it rides well. This bike has carried three adults (400 pounds total) without catastrophic failure. It is currently being tested/abused by the teenager pictured.
Step 1: Find a Bike
Find a bicycle for as cheap as possible then cut the seat-stays about here, and the chain-stays just behind the bottom bracket.
Step 2: Find Some Metal
Find some metal tubing. We found an old TV stand with strong tubing.
Step 3: Lay It Out
Lay it out and mark the tubing for more precise cutting. Aside from the TV stand tubing we also used a tube from a women's bicycle frame to strengthen the rear end.
Step 4: Build a Jig
I'm using the term "jig" very loosely. We found a piece of aluminium U-channel that was long enough and attached the frame to it in order to hold the frame in a straight line.
Step 5: Wire It Up
We used bailing wire to hold everything in place. Clamps would work better.
Step 6: Measure
We used string and a ruler and a nail on the end of the string to build a plumb-bob. The goal is keep everything as straight as possible.
Step 7: Measure More
Measure some more and cross your fingers.
Step 8: Find a Welder
We used this cheap stick welder and used 50-75 amps for all of the welding.
Step 9: Weld
tack weld then measure/eyeball everything for alignment and weld some more.
Step 10: Weld
Weld some more.
Step 11: Paint It and Name It
We chose the name Cheval which means horse in Haitian Creole. Choose a font, make a stencil and paint it on.
Step 12: Admire It
Our western style font is awesome.
Step 13: Make a Head-badge
Every bicycle needs a head-badge.
Step 14: The Whole Thing
We left out a lot of small steps because we used mostly found items and we lack some basic tools but hopefully this is enough information to get you started. A few things not previously mentioned are the board above the back wheel. To attach the board we pre-drilled holes through the wood and metal and then put nails through which we bent over underneath. We used fork blades welded to a metal tube as a support going from the end of the board to the rear axle. We added an extra chain to make it long enough but we are not using the large chain ring on the front of the bike so we made the chain long enough for the middle chain ring. We also welded foot pegs on the rear chain stays.
Most new rear derailleur cables should be long enough to reach, I didn't buy a new one but used a small bolt and nut to join a scrap of cable to the original to make it long enough. I did buy a new brake cable and it also works fine.
Step 15: Ride It
We are in Haiti so for $2.50 we bought the woven pannier which are made for a horse or mule. It looks great and works. The bike still needs a good kickstand.