You will need to cut up a bike frame for the main part of the trailer. Choosing the best frame for the job makes this easier. Choose a steel frame. You need to find a frame with this plate on it. This plate is found where the rear brakes attach. There should also be a plate where the kickstand attaches. You will have to strip the frame of all cables and gears. The seat and pedals are also unnecessary. The part where the pedal arms(cranks) attach is called the bottom bracket, this will become the pivot point of your trailer.

Save the wheels and front fork. You can use any wheel that fits, but I recommend using the same size as your bike uses so you only need to carry one size of tube when you travel.

The front fork is removed and kept. It should fit where the seat tube came out, and the seat post clamp should tighten enough to hold it in place.

There are special tools for working on bikes, I highly recommend finding a community bike shop near you to help with stripping the frame if you have trouble. The crank arms can be especially troublesome without proper tools.