What is the best process to add aeration to an existing pond to minimize fish kill? Do we need to wait until a different time of year?

Go very, very slow. I thought I was starting mine off slow, but I still lost a bunch of my largest BG, and that was back in May before it really got hot. Now, I run from midnight to 9 AM to minimize overheating during the day. If I had to restart mine now, I would probably start with 15 min a day for at least a week and then ever so slowly increase your run time. Again, "slow" is your friend.

There are a lot of variables that you will need to consider.

How old is the pond
How deep is the pond
how big is the pond
how many fish in the pond
What type aeration is best
Do you have electricity at the pond
Can you put it on a timer
How do you arrange the aeration, and at what depths
What is your water temps...

You either have a lot of reading to do, or hire someone that can design an aeration system for "your pond". Every pond is different.