In this article, I lay out the basic principles of jump starting a car's battery with jumper cables.
Disclaimer: I'm not a mechanic. So, please don't follow the instructions in this article without consulting a professional mechanic.
I'm merely noting the steps down as a quick reference I can get to whenever needed.
Prerequisites
Dry hands stripped of any metal objects (also wear gloves if possible)
Jumper cables - 1 red (positive) and 1 black (negative)
A healthy vehicle that's either equally as or more powerful than the dead car
Position the vehicles so that their batteries are close enough for the jumper cables to reach
Make sure the vehicles are not touching at all
Make sure both vehicles are turned off before jump starting
Safety basics
Car batteries release oxygen and hydrogen. One tiny spark close to the battery could cause an explosion. Even if the probability of this happening is low, I think safety can go a long way here.
Modern cars come with complex electronics. It's best not to use them for jump starting dead cars as that might mess up their electronics. The better alternative is a battery booster.
The following order must be followed to reduce the risk of sparks near the battery.
Connect
Red Dead - connect the red cable to the dead battery's positive terminal
Red Healthy - connect the red cable to the healthy battery's positive terminal
Black Healthy - connect the black cable to the healthy battery's negative terminal
Black Dead - connect the black cable to the dead car's engine block (a bolt or piece of metal sticking out on the engine block)
Turn the healthy car's engine on. Then, try turning the dead car's engine on. If it doesn't, accelerate the healthy car's engine to 2000rpm and try again. If it still doesn't work, it means the battery is gone and probably needs replacing but it's best to have a mechanic check.
If the dead car's engine is able to start, keep both cars' engines running.
Disconnect
This phase is the exact opposite of the connect phase.
Black Dead - disconnect the black cable from the dead car's engine block
Black Healthy - disconnect the black cable from the healthy battery's negative terminal
Red Healthy - disconnect the red cable from the healthy battery's positive terminal
Red Dead - disconnect the red cable from the dead battery's positive terminal
At this point, you can turn off the healthy car's engine only. Do not turn the dead car's engine off. Drive the dead car for around 20-30mins to allow the alternator to charge the battery fully.
Why the engine block?
The fourth step in the connect phase instructs you to connect the cable to the dead car's engine block, not the battery's negative terminal. It's basically the same thing since the negative terminal is connected to the car's body, acting as ground. However, when connecting to or disconnecting from the engine block, sparks are away from the battery, which is safer than the battery's terminal. You can alternatively use other ground points on the car that are away from the battery.
Leaving the batteries connected for a long period of time with both engines off could cause the healthy car's battery to discharge. This means needing a third car or battery booster to jump start it.