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When connecting your battery, connect the positive (red) first and the negative (black) second. When removing the battery, disconnect the negative (black) first and the positive (red) second. Been doing this for nearly 30 years. ALWAYS connect the positive post first.
I will call the battery that is not in the car the external battery. First connect the positive (+, usually red) jumper cable to the positive car battery cable, & then the other end of that jumper cable to the positive external battery terminal. Then do the same with the negative (-, usually black) car battery cable & jumper cable.