Welding exhaust bung in pipe help.

I'm going to weld the bung this week and hook up the Innovate AF meter. I can weld but, this is something I've never done. I have a few questions since I'm getting conflicting info from my searches. Thanks

Can someone point me to a how to that works or instruct me how to?
Should i drill the hole ID or OD of the bung?
Do I need to flatten the pipe or grind the bung to match pipe?