PC-AT Cable and keyboard socket.
(The picture shows a PC-AT style cable. The method below describes how to
make a PS/2 style cable, which is the type used with current motherboards)
Go to a computer shop and buy a PS/2 keyboard extension
cable. You know, the type which is 3 meters long and has a PS/2 plug at
both ends. These cables are quite cheap; probably cheaper than buying one
PS/2 plug and the cable separately.
Anyway, cut off the female plug from one end and strip
the insulation. Use a multimeter to work out which color is which pin.
Then you can see from my diagram where to connect each color. Open the
keyboard and solder the wires to the pins on the circuit board where the
RJ socket is mounted. You can even remove the RJ socket entirely and use
the hole in the case to pass the cable through.
PS/2 and motherboard wiring reference.
PS/2 connector wiring reference.