Read the whole guide before flashing, to make sure you've understood everything correctly.
Flashing images with fastboot is needed only if you're not already on a lineage build. If you are, you can just reboot in recovery and go to the flashing ROM zip step.
Download boot, dtbo and vendor_boot images from the download link
To get the path you can usually just drag and drop image to terminal/console.
Reboot to recovery and do a factory reset (Factory reset -> Format data -> Format data)
Flash the ROM package with adb sideload by doing the following:
In the recovery enter the menu "Apply update" and then "Apply from ADB"
In the PC, run from terminal adb sideload /path/to/lineage.zip
When the flash is finished, reboot to recovery when asked if you want GApps. The flash percentage will stop at 47%, don't worry about it.
Flash the GApps package with adb sideload (optional). Just follow the same steps you did earlier for the ROM, but with the GApps zip package. If the package verification fails, don't worry about it. Just press Yes and make it continue in the flash.