Which one is the positive nipple?

The important thing is not to complete the circuit near a battery, since it can cause sparks which can ignite the flammable vapors that batteries tend to emit when discharging. This is especially important near the dead battery, which has potentially been discharging quite a lot recently.

So the usual sequence is:

1. Dead positive
2. Live positive
3. Live negative
4. Dead ground (to the metal chassis, ideally not right next to the battery)

I usually start by clipping the dead negative cable to something non-conductive to keep it out of the way and then move it to ground as the last step.