How do I calm down a toddler when they are having a tantrum?
Toddlers have big expectations and even bigger feelings and they do not yet hav the ability to identify and control their emotions and reactions; it's the parents job to help their toddlers learn to cope and adjust. Not all children are the same, so there is no "magic bullet" to calming down a child in a tantrum, but a couple things that can help when your child is having a melt-down:

1. Remember that their feelings are real and they are not trying to manipulate. Young children do not yet have the capacity to manipulate
2. Make sure the space is safe for the child, and any other children around.
3. Do not raise your voice to match their intensity! Use a calm, level tone with the child. Though this can be very difficult when you are also frustrated, a lower tone will help the child lower theirs as well.
3. Let the tantrum run out of steam. 
4. Give a lot of hugs to your upset child, when they are ready
5. When your child is calmed down, talk about what happened! Give their feelings names (sad, angry, frustrated) to help your child identify the feeling when it happens next time, and talk with them about what they should do instead of throwing a tantrum (take a break, tell an adult/parent, deep breaths, etc.)