I’ve downloaded Mint to my Mac, but cannot figure out how to verify the sha256 sums. I can get to the terminal screen, but when I type in the commands given on the installation help page for Mint (sha256sum -b yourfile.iso), nothing happens. Help!
Check this out as a cross platform utility … https://gtkhash.org/ I have used it many times before being comfortable using terminal …
I posted about this utility and how to use it here … Old IBM T42 32bit storage locker find .... or how fast can I install Linux on this Oldtimer
I’m afraid that even that is too technical for me. 
I tried using linux a few years ago and also got stuck here, unable to verify, and gave up.
When I tried to download the gtkhash, my computer told me there wasn’t an application on the computer that could open it.
Ok, somehow I got the integrity check to work, but now I can’t figure out how to do the authenticity check with the gpg command. It tells me that the gpg command not found.
How essential is it that I do this check before installing Mint?
If you have created a USB key with Mint Installer you can start up that and use the live environment (Try out Mint) rather than installing it. This way you can see Mint running on your machine and follow the directions you have from inside Mint. When you are ready you can invoke the installer with the desktop “Install Mint” icon.
It is a very good idea to check the file is as the vendor created it. For most activities it is not necessary to do terminal work like verifying checksums. It is great to get comfortable with the terminal console as it is a very powereful tool to maintain your system at peak security and performance.