When do you put in the old SIM card in the new phone

Whe do you put in the old SIM card into the new phone?

...when you are ready to use it.

I presume you are asking about a new iPhone that has been set up with data ("restored") from a retiring iPhone?

A SIM isn't important in setting up the new phone provided you can connect to the Internet via wi-fi. I recently gave a hand-me-down to a family member but I first had a few days work to set up her photos for iCloud, a task she had avoided despite my prodding. I set it up as her new iPhone using these instructions, then sorted out her iCloud Photo syncing. It was only after I'd done that for her that I declared my old iPhone her new iPhone and put the SIM in it. She had been using her retiring iPhone with the SIM during this period.

In effect she had two iPhones on her iCloud account; the old one and the one I was working on.