All you need is a test light. No need for that thing.
But, the pics show it connected to the current inputs of a meter and the setting on DC current. Disconnect the - battery cable and connect the leads as shown in the pic.
This is one way to check.
Turn the key on.
Does the meter show current flowing?
Check til the meter reads zero.
Do you have a stereo with memory? Disconnect it. Check
Reconnect things once checked.
Disconnect the alternator. Check
Disconnect the regulator. Check
Pull one side of a fuse. Check
Repeat the last step til there are no more fuses.
Or just use an amp meter, pull the fuse and touch each receptacle.
It will tell you if there is any drain.
Make notes so you know if there is ever a change.
Most circuits except a newer radio should have zero drain when off.