The Black wire will connect to body ground, and it should be a good solid ground and if this wire is not grounded well then you might receive a primary voltage shock if you touch anything that is metal or grounded inside of the vehicle when the engine is running.
The Red wire should be connected to power when the key is in the "Run" position only, and a good place to find that source is one of the "IGN" connections at the fuse panel using a covered female spade connector. Use a test light and be certain that the connection only has power when the key is in the "Run" position, and has no power when the key is in the "Off" or "Accy" position.
The White or Yellow wire should connect to the "LPS" connection at the fuse panel next to the LPS fuse using a covered female spade connector.
The Green wire will connect to the "-" negative side of the ignition coil or the primary side of the ignition coil, and for later model GM HEI equipped vehicles this will be the "Tach" connection next to the main "Bat" connection on the distributor cap if the ignition coil is mounted inside of the distributor cap, or to the "Tach" or "Test" lead that runs from the negative side of the ignition coil if the ignition coil is mounted separate from the distributor. To locate the lead run down the wire from the negative side of the ignition coil and the lead should break off from that wire and it should have a tag on the wire that will say either "Tach" or "Test" (sometimes it is just an empty black wire connector) and that is if the lead does not come straight off from the wire connector at the ignition coil. If there is no lead you can get a wire tap connector to tap into the wire from the negative side of the ignition coil and then use a covered male spade connector to connect the Green wire from the tachometer to the wire tap connector.
Here is a diagram to help assist you although it might be hard to make it out, and let me know if you require any further assistance.