1.Download Linux Mint and a linux to usb software
2.create a bootable usb
3.connect the HDD for the Linux OS
4.boot from USB and select install
5.select the proper HDD
6.After installation complete restart PC
7.go to BIOS and select your primary HDD(OS) as first bootable option
If you want to boot from a particular OS on any ocaccion when you start your PC go to boot selection (depending on your board the key to press changes, mines is f9)
I will recomment to disconnect you Main OS HDD before installing Linux Mint just for precautions.
Personally I have Windows 10 and a second HDD with Ubuntu.