Hi there,
Personally, I have found the tutorial from the official R-Pi forum to be very useful: STICKY: HOWTO: Move the filesystem to a USB stick/Drive - Raspberry Pi Forums
Note that this still boots from the SD card (kernel is there) but the rootfs will be located on the HDD. It is also possible to burn some fuses in the R-Pi firmware and make it fully boot from an external HDD entirely but this is a one-way path AFAIK: you will lose the ability to boot from SD after that IIRC.
I second @wbk: if your install is new enough, it’s probably easier to install on the HDD directly.
Good luck!