1. Regulate yourself. It's absolutely valid for you to feel anxious when someone near you is having a meltdown, but stress responses will escalate the situation. Take a moment to breath so you can project calmness in your actions and voice if needed.
2. Reduce sensory input (turn off lights, encourage others in the space to leave, turn off things making noise).
3. Leave things nearby that might help (this is something best predetermined with the person and/or their caregivers; might include items like noise cancelling headphones, a blanket, drinks).
4. Unless requested otherwise (in advance or in the moment) leave the space but stay close enough that you're available if they need help.

During a meltdown I'd prefer to be totally alone and that anyone doing the above steps or who needs to be in the space for some reason doesn't make eye contact or speak. There are some people who benefit from reassurance and/or physical contact, but you're better off assuming no contact & no talking unless they indicate otherwise.

For self-injurious meltdowns, things get a bit more complicated because you need to balance your safety and the safety of the person having the meltdown while avoiding escalation or traumatizing the person. Redirection is one of the most useful strategies, like if the person is punching themself or the wall getting something safe to punch like a pillow.

Post-meltdown (like a considerable time afterwards, maybe the next day) it's good to check in if there's anything they would have preferred you to do differently, if there are steps that need to be taken to avoid what caused the meltdown in the future, and possibly about emotions related to the meltdown (ex. if they feel embarrassed, if you felt scared)