When installing a car battery you should connect the positive battery cable first and then the negative battery cable can be connected.
This should be done to prevent arcing of the battery if you were to make contact with the wrench from the positive battery terminal to the ground of the battery or cars body.
That would then create a short and cause arcing.
So connect the positive battery cable on the battery first and then connect the negative battery cable.
When disconnecting the battery cables on a car batter you should remove the negative battery cable first and then the positive to avoid the arcing situation as well.