First you must open the button. To do this, you insert a small screwdriver into the side where the button cap is exposed. It’s similar to this, although this is a Sanwa button:
After this point, it’s generally good to remove the microswitch by pushing it up. Again with the Sanwa button, but it is the same process with Seimitsu:
Then, you would take the piece in the middle and turn it over, and drill holes into the back. If you don’t have a drill, some people have poked holes out by heating up that small screwdriver they opened the button with, and then using it to poke a hole in the back.
After this point, solder on two wires to the VCC and Ground points. Two different colors would be helpful, so that you can identify which wire is soldered to which.
Pop the microswitch back in, and then insert the KNSert into the button. Be sure to thread the wires through the holes you drilled out.
Now, note, the labels for VCC and Ground are backwards, so you’ll have to flip the wiring around:
Connect the VCC (Which is labeled as GND) terminal to a 5v power source, and connect GND (Which is labeled VCC) to its respective ground. You will need a resistor somewhere in there. You can cut the trace of the KNsert and solder your own on, or just connect a resistor between the power source and KNsert, or between KNsert and Ground. In an arcade stick with a USB cord, a 5v source can be acquired by connecting to the red USB wire, while Ground can be connected to the black USB wire. You can use lower voltages than 5v, but the LEDs will be dimmer.