It's quite simple really, just need a relay about £3, some wire red and green of 14/030 and 28/030 gauge(or similar), and a fuse holder.
Just a case of basic wiring from the relay to the fan switch and from the relay to the fans, then just a earth for fans and relay, put in fuse as close to battery as you can get.
The parts are easy to get from auto electrical places, or car shops(more expensive) or online/phone vehiclewiringproducts.co.uk
tabetha