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Reinforce the joint of the connector to the EIB with epoxy. Without this step, the solder connection can break after a few connect/disconnect cycles. Be careful to avoid getting any epoxy into the vias, as small amounts can be very hard to spot and can cause connection problems when loading electrodes later.
(optional) connect GND to REF or connect two REFs and GNDs
On many EIBs, there is a built-in way to tie REF and GND together with either a solder bead or by briding two vias with a small piece of wire. In some cases, they might be tied together by default and a trace needs to be cut to separate them if desired.
On EIBs with two connectors that have independent GND and REF connections, it might make sense to connect these together by feeding one wire through ground&ref (or separate GND and REF depending on application) wire through the appropriate vias and soldering it. This results in two free ends to the wire, one for the shield, the other for the GND screw with minimal soldering etc.
The EIB is now ready to be glued onto the drive body.