Enable developer options By going to

Settings > System > About Phone > Developer options (Tap 7 times)

Enable OEM unlock in Developer settings

Reboot into recovery

If your one Verizon you may need to go through a couple extra steps to get the oem unlock to show up in the developer settings. More info here

Boot into twrp

fastboot img twrp.iso

Select the option in TWRP to sideload and sideload the Lineage iso

adb sideload lineage

Reboot install GAPPS

adb sideload gapps.zip