Asking How to wipe old iMac after migration to new iMac

Hello. My first iMac lasted me a whopping fifteen years. Since it is showing signs of diminished dependability, it was time to upgrade. The migration tools worked perfectly and I am happily using the new iMac.

What I want to do is effectively remove all of my data without compromising anything that is in the cloud.

I restarted the old iMac - turned off iCloud and wifi and bluetooth. And now I want to ask the forum "what is the smartest way to clean up this old computer before it goes away?

Thanks in advance - FormerNerd