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 metres 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 colour is which pin. Then you can see from my diagram where to connect each colour. 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 connector wiring reference.