how do you get certified to be a passport photographer?

In the US, anyone can take the photo, including yourself.

I did this. It's surprisingly hard to find a solid white background and get uniform lighting at home. Took many shots.

In the U.K. people used to go to a booth, but nowadays you just get a well lit white wall and take a selfie on your phone.

Fifteen years ago I did my own in Canada, and just wrote my own name and phone number on the back as at the "photographer". They gave me the hairy eyeball at the passport office though but let it slide since the pics did meet the requirements.

After that I got them done at the local framing shop.