This is the most difficult part and depends on the electronic hardware you have. please read this step carefully.
The basic idea is that you need to solder a wire between the WiFi board's RF output and the driven element of the yagi antenna.
But USB WiFi modems come either with an internal antenna or with an external antenna. Those with external antennae, like mine, are easier to connect because you are just replacing the external whip antenna with the Yagi. Those with internal antenna may need to have their on-board strip antenna modified as illustrated in the pictures here. You need to slightly experiment in this case.
I have tried soldering a coax to my board's antenna connectors and the two ends of the yagi's active element loop but it did not work in my case. I have no explanations why that did not work, but other DIYers that have built Yagi antennae connected their antenna in this manner.
In my case, I just connected a single thin strand of copper wire between the active element of the strip antenna and one end of the loop of the driven element.
Please read the annotations of the pictures for more details.