The idea came for this the first time I saw one of the "tag along" bikes for kids. I liked the idea of having a trailer that tracks with the bike. I ride on public roads and with a two wheeled trailer there is always concern about making sure of the position of the trailer on the road. This is just over two feet wide and tracks directly with the bike, eliminating that concern.
I like to make use of things I have and don't really like buying a lot of stuff to complete a project. The majority of what I used was stuff on hand. I also build things off of ideas, I like the process of designing and solving how to put something together. This instructable shows the major steps I took.
I have been watching Craigslist for a tag along for a while. Most of the ones listed were around $40, reasonable enough considering new ones are more than twice that. I found one in a thrift shop for $15. It was missing the hitch so I had to build one. I included that in the instructable but if the unit you use has one great.
First step is obvious, strip off unwanted items - handlebars, seat and pedels. Simple hand tools will do the trick for all of it.