I just bought a house, and consequently had to move all my stuff. In order to move all that stuff I had a pretty simple choice- I could move via car/truck/moving van, or I could try and do it via bicycle. Now I know this is border line pretentious sounding but I was honestly just looking for the next big challenge/project/mind occupier and nothing more. Anyways, decided to try my hand at building a bike trailer.
Lucky for me, buddy Scott had been getting seriously good at welding steel (he's been making bike frames, and making them well). So I hit him up for some welding in exchange for beer and pizza (and the challenge of a DIY steel project). So I pieced together the most necessary items by checking craigslist relentlessly and finally breaking down and just buying some of the needed tubing. It didn't turn out to be much really, just some square (1"x1") stuff, ~24 feet of it I think; and a rod bearing end, to allow lateral and vertical movement at the joint; oh, and some eyelet bolts, to hook bungies/straps to. I already had two 26" mountain bike front wheels; salvaged a few right angle steel sections; and dumpster dove for a giant sheet of plywood behind the Home Depot. All in all, one of the cheaper projects to undertake.
With all the materials and a garage full of the right tools, Scott and I embarked. We started with the determination that the bed would be a good 8'x4' platform, with the wheels set within square wells. The hitch system was undecided at this point, but figured it would buff itself out. So we measured, cut and sanded the pieces of steel to size, arranging them on the ground in the shape we wanted. The wheel axle offset was something I hadn't thought through entirely, so we just pushed it back towards the back by a foot. Then Scott tacked, then welded each of the joints.
The welding went as smoothly as we could have hoped, like I said, the dude's got skill. So with the frame together we just had to pop the wheels into the wells and work out the hitch. The first iteration failed pretty miserably, days after the undertaking. The load put too much pressure on one point which was weak and it buckled and broke off. Then we tried a welded system which also bit the dust after loading down pretty heavily and trying to ride over some railroad tracks...
But the third iteration, well actually I haven't built that yet, but I did get away with the move (well just over half of everything) before V2 blew up. Here's to the next one!
I realize now this could have been a little more informative as to the actual steps involved, but then again there are plenty of how-to guides and videos out there just waiting for you to check them out!