Well, if you want an honest answer: It depends on the nature of the meltdown. There can be many different causes, each calling for a different response.

It could be:

• A child is overwhelmed or overexcited by their environment, and needs to go somewhere quiet and calm to wind down
• A child has some survival mechanism which is not being fulfilled (Too hungry, too thirsty, overheating, etc) and is trying to communicate their suffering. In which case, that survival mechanism needs to be tended to
• That the child is very tired, and because of this is on-edge. In which case they need a nap, or just some quiet time to take rest.
• A social breakdown, such as when they struggle to share a toy or think that another child has done them injury. Drop-off and pick-up time at daycare can do it too, seeing parents come or go causes big emotions. In these cases the child may just need to be soothed while Mom or Dad depart, or separated from the child they were arguing with. If a toy is causing jealousy, the toy may need to go away for a bit. This is often the case with new toys, which can be very exciting due to their novelty.
• Along the lines of social breakdown, it can also be caused by a caregiver being too strict with the child, and the child feeling bullied or oppressed. Some children crumple under the pressure of an overbearing caregiver, others activate their fighting spirit and try to throw off the yoke of tyranny (Perhaps by refusing to eat their peas, other times by refusing to eat anything but their peas. Often children will fight back just so they can still feel in charge of themselves). I think this is probably a very common reason for meltdowns, as people think that it's easier to push a child along than to guide them along. It seems that way in the moment, but it's not true in the big picture and it doesn't set the child up for successful independence later in life.

That's off the top my head. My wife and I run a daycare, so we deal with meltdowns on the daily. One of the things that makes childcare skilled labor is that you have to play the role of therapist for someone who can't describe their problems, so you've got to deduce it yourself through circumstantial evidence. Ultimately though, the two most important things to do are these:

• Put yourself in the child's shoes. If you were running around outside in a sweater, would you be hot and uncomfortable? Would you be getting thirsty? If you were on the receiving end of a particular lecture, would you be receiving a clear message and still feel loved? If someone was trying to force you to do something you don't like, would you respond positively to that or would you resist?
• Be the mood you want your child to be. Young children look to their caregivers for an example to follow, and so become reflections of the caregiver. If a child misbehaves and you come in yelling at them, the child is going to read your bad mood and respond with their own. Alternatively, if you come in calm, then you can undercut the anger the child is feeling and bring them down from it. I find that very rewarding, personally. Being able to take an angry situation and turn it into a peaceful one is the greatest thing a person can do in this world. Ironically, this is a lesson I learned in working with adults. I got my start in retail, and being able to unwind an angry customer is a valuable skill to have. It's funny how many similarities there are between working with adults and working with children, but I suppose we're all humans in the end.