How to remove hard drive from iMac?

To remove it click the Do-It-Yourself videos on https://eshop.macsales.com/installvideos and then find your exact model and follow the instructions. However if you simply go to Apple Recycling Programs and use that not only is it free (no tools needed to disassemble the computer) any data on the HD will be destroyed.

If the iMac is still functional & you don't want to reinstall the OS & it contains a hard drive, then you can create a bootable ShredOS USB stick which can allow you to write zeroes to the internal hard drive. This can be easier than trying to find the "Secure Erase" option within Disk Utility. @LexxDataRecSolutions has you covered with the macOS native options.

A1312 would be a 27-inch iMac from Late 2009, Mid 2010, or Mid 2011.

Those had user-accessible RAM (hatch on the outside), but their hard drives were buried inside, and considered non-user-accessible.

We can not post tare-down or repair guides here in this forum.

If you are unable to erase the iMac for recycling...?

as per What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

Then you're best bet, is to google search for on-line guides like iFixit.

Load your Mac in Internet Recovery Mode, start Disk Utility, format the system disk, and select 2 or more passes in Security Options. Install fresh macOS and enable FileVault during installation.

As a final step, you can run a scan of the system disk using any data recovery utility, such as Disk Drill, UFS Explorer, etc., to ensure that nothing can be recovered. Alternatively, you can remove the disk from the Mac (which will not be very easy) and drill a couple of holes through it :-)