How to downgrade to macOS Sierra without Time Machine
I got it from this link.
IMPORTANT:
DO NOT downgrade using macOS Sierra (10.12 Installer) disk utility to Erase your disk, you HAVE TO use the Online Recovery tool (or High Sierra (10.13) Disk Utility) to Erase your disk from APFS back to HFS+ , then restart and use your macOS Sierra installer from USB.
If you use Sierra to Erase disk, you'll get stuck in a Reboot Loop which is what happened to me.