Run them full speed directly off the PSU, or if you want to control the speed, use a fan controller. (You will also see reference to a 'seven volt wire' - this is where the fan is connected between the 12V and 5V rails giving 7 volts, although I'm not sure how that woould affect PSU regulation as you are cross connecting two ostensibly independant rails - but I'd need to see a cct diagram to be definitive about it - personally I wouldn't do it)