Most electrical gauges, like temp and oil pressure have 3 terminals on the back.
One is a positive terminal that you key hot ( only has current when the key is on) the second one is a ground, that you run to a chassis ground. And the remaining one is "S" for sender. That wire goes to either an oil pressure sender that you screw into an appropriate oil galley in the block, or if it is the water temp gauge, you install a temp sender in the engine coolant chamber somewhere. You will want to get matched senders and gauges, as the resistance varies from brand to brand.

Don