On the end of the electrical wire coming from the appropriate oil pressure sender that came with the guage....
Seriously, you can either make a hole for it in the blank dash, mount it under the dash on the right side of the column or make a new centre console that houses it... For the oile pressure sender - keep the original one in and buy an extra T piece and them plumb both the original and new oil pressure senders into the same location that the original one was in....
Quite easy actually, although I mostly fit mechanical oil pressure guages as the only electric one I ever fitted plays up and I have never had a failure of a mechanical one...