Cut off the barrel connector from the AC adapter, and strip the red and black wires inside. Slide a medium diameter piece of heat shrink tubing over the whole wire, then put a smaller piece on each of the two wires.
Grab the input power coil (more square circuit board) and with the help of a 3rd hand tool, solder the red wire to the AC adapter's red wire, and likewise with the black wires.
Use a heat gun or lighter to shrink the smaller pieces of heat shrink over the solder joints, then the bigger piece over the whole connection.
Alternatively, you can plug your AC adapter into a terminal block connector and use a screwdriver to secure the circuit board's wires to it. This eliminates the need to solder, but only on this half of the circuit.
Reference the above circuit diagram when building this project! The coil attached to the more square circuit board attaches to the incoming power, in this case from a 9V AC adapter. The other, more long and skinny, circuit board gets soldered to the USB connector as shown.
The red and black wires affix to the outer pads on the USB connector as shown above. At this point you may wish to lengthen the wires attached to the long skinny circuit board to give your USB cable some slack inside the bag. When you've got wires of your desired length in place, slide on the strain relief collar that comes with the USB connector.
With the help of a 3rd hand tool, tin the pads and wires separately and then reheat with your soldering iron to make the connections.
It's time to test your circuit! Plug the AC adapter into the wall and use helping hands or other things on your desk to orient the coils as close as possible to one another. They must be coaxial (parallel).
The coils should be 3mm or closer for most efficient energy transfer.
With the coils in place, use a multimeter to test the output on the USB end. It should read 5 volts.
Plug the naked USB connector into your phone and check that it charges. Some phones have more finicky charging requirements than others, so if you have a successful multimeter reading but your phone won't recognize the charge, you might consider charging up one of our backup battery packs instead (which can then be used to charge your phone).
Close up the USB connector by placing the port and strain relief collar in one half of the plastic shell.
Clip on the other half of the shell and your USB cable is complete!