If you can still access TWRP recovery, you should just simply use TWRP recovery to install stock firmware as OnePlus 6 stock firmwares are fully-compatible with TWRP. But if for some reason you cannot access TWRP, you can go ahead and use the fastboot method. Now, the fastboot method works very well and will get you out of jail in many unfortunate situations due to new file partitioning system on the OnePlus 6.
Step 1. Power off your OnePlus 6.
Step 2. Make sure your USB cable is DISCONNECTED! (as if it is connected to your computer it will enter a hard-brick unroot mode)
Step 3. Hold down Volume Up and Power buttons together until you are in Fastboot mode.
Step 4. Connect a USB cable from your OnePlus 6 to your computer.
Step 5. Unzip the stock firmware zip file you downloaded.
Step 6. For Windows, simply double-click on flash-all.bat and wait 10 minutes. DO NOT TOUCH ANYTHING!
For Mac or Linux, change all file permissions to 755, then run flash-all.sh.