Output of alternator to amp meter. Other side of amp meter to battery hot cable. Battery connection is often made at the starter solenoid/switch.
Light and ignition switch inputs connect to alternator side of ammeter.
Connect ammeter so it shows discharge (--) with engine off and switches on. If it shows + then reverse the ammeter connections.
I use 10 gauge wire from alternator to ammeter and from ammeter to battery hot.
Unless modified alternators are negative ground