Go to TWRP format all, factory reset all, wipe all,
then
Go in TWRP > Advanced > ADB Sideload
Create a folder called 'rom' and put it in your Downloads folder. ( On PC)
Put your downloaded OS in it and rename it to rom.zip.
1. Start cmd.exe
2. Type cd Downloads
3. Type cd rom
4. Type adb devices and make sure you see your phone
5. Type adb sideload rom.zip