If you mean for a dash mounted ammeter, it connects as Wildgoody said. You disconnect the wire between the alternator and the battery and connect the ammeter where the wire was. When you start the engine and the battery is charging, the meter should move to the "+". If it moves the other way, swap the connections on the back of the meter, or under the hood, whichever is easier. Incidentally, unless you like to have maximum sparks when you don't expect them
, disconnect the ground lead on your battery while disconnecting the alternator wire, installing the meter or switching leads.
When the battery is low, the meter might read 25 to 30 amps, unless you have a larger alternator. The reading should go down to a few amps as the battery is charged.