2012 Mac - how to destroy hard drive

If the iMac still powers on see: What to do before you sell, give away, or trade in your Mac - Apple Support

For added security you can choose to perform multiple erase/overwrite cycles.

[Link edited by Moderator as following the procedures at the site may lead to damage to the user’s device]

Actually if the Mac is using a hard drive (or Fusion Drive) as opposed to just an SSD, then the simple erase instructions will still leave your data on the hard drive unless the hard drive was encrypted with Filevault before erasing it. A simple erase for a computer using an SSD is sufficient due to how SSDs work, but hard drives retain their data until that data is overwritten. A simple erase will only repartition the drive and put down a few items...the bulk of the hard drive is untouched.

The easiest method to securely erase the hard drive may be to try using ShredOS. It will allow you to write zeroes to the whole hard drive. If you have an SSD, then this is not necessary. If you had a Fusion Drive, then just securely erase the hard drive portion and leave the SSD portion alone. You can use the download ShredOS .img or .iso file as a source for Etcher (Mac/Windows/Linux) to create a bootable ShredOS USB stick. You can use the 64 bit version of ShredOS on a 2012 Mac.

I use old school methods: several 1/4" drilled holes. Sometimes a 5.56 NATO.