The standard recording configuration is to short the external reference with the ground, and connect these to a saline bath (or agar) above the craniotomy for acute implants, or connect them to a skull screw for chronic implants.
The soldering notes below are primarily for Phase3A probes (which version do I have?); for Phase3B, solder sites on the 'wings' of the flex cable can be used instead. For an approach with Phase3B, see this video.
Important: when soldering on the probe, it is critical to take care not to heat the probe too much. For this reason, it is recommended to use a non-standard method for the soldering suggested below: take a drop of solder on the tip of the iron and then touch it to the spot on the probe.
One scheme for shorting reference and ground:
One way to connect to the gnd conveniently is at the headstage, at the gold circle labeled below. If gnd and ext ref have been shorted with soldering as above, then it is not necessary to attach a wire to the probe base at the point of soldering, since this shorted line may be accessed at the headstage.
Internal referencing is available for selection in software. However, tests using this have been mixed, and this author cannot promise you that there is any particular configuration in which this will work. Whether you are using internal or external referencing, the sites designated for internal referencing will not be recordable (they will appear to produce data but it is junk).