You need to download & install the installer from LibreELEC Downloads – LibreELEC
on the same page download the generic image, Leia, *.img.gz
Use the creator to burn to a usb stick.
Insert usb, boot from it, then choose which hard drive/ or second usb you want to install it too
It will tell you to remove the installer usb, then on reboot, it should reboot to the drive you installed too
Follow the wizard, for the initial install, use a usb keyboard or remote