I've gone on before on ManMade about my love of simple machines that work well, and that the bicycle is the most perfect and useful example. And for years, cyclists have been trying to figure out safe and efficient ways to move more than just human bodies from one place to another. Backpacks, frame bags, panniers, baskets, etc, all work well, but what about seriously larger-scale stuff that needs to stay off the frame lest it interfere with the trip. That's when you need a cargo trailer.
Mike Cheung from Tinkering Monkey took a stab at building his own on the cheap, using the frame from a Schwinn child-carrying trailer and outfitting it with rigid sides, and a new, more solid wooden arm that attaches similarly to a tractor trailer.
Mike's post is more of a narrative about his process of creating the trailer than a straightforward how-to. So, if you're looking for actual step-by-step details, Instructables has a whole collection of DIY articles and efforts on the subject.
How to Make a Cheap and Easy Bike Cargo Trailer [TinkeringMonkey.com]