Hi Jethro,
If your guide camera doesn't have a guide port (e.g. using a webcam), then use the GPUSB to connect to the mount using an RJ11 6P6C cable from memory.
Diagram:
https://2.bp.blogspot.com/_BCg7Ox9nm...oguiding+3.jpg
Otherwise if your guide camera does have a guide port, then you get connect it directly to the mount, as per this diagram. An advantage of this method is one less cable from the computer.
https://4.bp.blogspot.com/_BCg7Ox9nm...oguiding+2.jpg
Either method doesn't give full computer control (e.g. goto, use with planetarium software etc) but will allow guiding. If you want the full control route then a EQDIR cable as per Jeremy's recommendation is the go, or you can buy/make a serial to RJ45 cable to connect to the Skyscan handcontroller which then connects to the mount like this:
https://4.bp.blogspot.com/_BCg7Ox9nm...oguiding+1.jpg
Cheers