Prerequisites:

Complete Download

• Before flashing, make sure you have downloaded all the necessary files. The main tool used for flashing firmware on the OnePlus 5T is the MSMDownloadTool and, you will also need to download the correct firmware update for your model of the OnePlus 5T. You can download the firmware file from the official OnePlus site or trusted community links.

Install the Required Drivers

• Make sure that you have installed the proper USB drivers so that you can establish the connection to your PC. These drivers enable your computer to detect the devices correctly and allow MSMDownloadTool to operate smoothly. The proper drivers can depend on the official OnePlus support site.

Backup Important Data

• And of course, back up any important data as flashing the firmware will wipe all user data, settings, and apps from your device. To avoid losing any personal data, you can back up files to a cloud storage or move them to a computer.

Make Sure Your Battery Has Plenty of Charge

• You will also need to make sure that your OnePlus 5T has at least 50% battery before going through the flashing process. If power is lost while flashing, this can corrupt the system and render the device unbootable, amongst other scenarios.

Flashing Process:

Extract Firmware Files

• The necessary files will need to be extracted after download:
• Find the downloaded firmware package and MSMDownloadTool.

Just make sure you have all the required files before going to the next step.

Entry to the Download Mode (aka EDL Mode)

• First, turn off your OnePlus 5T completely.
• Connect your OnePlus 5T to your PC using a USB cable.
• Immediately press and hold the Volume Up key.
• Continue holding until it goes onto EDL (Emergency Download) mode.
  If the device is not entering EDL mode, repeat the process with a steady USB connection.

Run MSMDownloadTool

• Your OnePlus 5T is in EDL mode, so go through the following steps:
  Locate the extracted firmware folder on your PC.
  Find the MSMDownloadTool.exe file
  To use the tool, right-click the executable and choose Run as Administrator.
  This will give the tool the proper permission to communicate with the device and flash firmware correctly.

Load Firmware into MSMDownloadTool

• Click the button “Load” or “Browse” in MSMDownloadTool.
• Open the folder by clicking on the path where you extracted ROM files.
• Download the relevant firmware file for your OnePlus 5T.
• Make sure the proper firmware is loaded before moving on.

Start the Flashing Process

• Inside MSMDownloadTool, click on the “Start” or “Download” button.
• The flashing process should now start automatically.
• Keep your device connected and do not turn it off during this process.
  MSMDownloadTool will show the flashing state in the progress bar at its interface. Based on the efficiency of your PC and the size of the firmware, flashing can take a few minutes.

Wait for Completion

• The flashing will continue, and you should see progress on MSMDownloadTool.
• MSMDownloadTool should show you a message like “Download Complete” or something similar after it successfully finishes the process.
• Make sure that the process is 100% before going ahead

Unplug the Device and Power-It-Down

• Finally, detach your OnePlus 5T from the computer after the flashing is completed.
• Force restart the device by pressing the Power button.
• On the first boot it may take longer than usual, just wait.
• If booting goes fine, continue with the initial setup process, and from that point, reconfigure your device.

Important Notes:

1. Go For Firmware Compatibility Check: Installing incorrect firmware equipped with the wrong version can lead to serious problems like a boot loop or soft brick. Make sure the firmware you are using is specific to your OnePlus 5T model.
2. Note that this will destroy ALL DATA on your device. Before doing so, please ensure you have backed up any important files.
3. On the Flasher Battery: Avoid sudden shutdowns within flash chips with more than 50% battery charge on the device.
4. Risks of Flashing: Flashing improperly, a device can become unusable, and doing so can void your warranty. Use at your own risk, and only if you understand the risks fully.
5. Please note: Ensure to refer to official OnePlus documentation or community forums for any further queries and troubleshooting notes.

Final Considerations:

Always refer to any official documentation available as these steps serve as an overview of what to expect in the process and can vary depending upon the OnePlus firmware version or update. To help you troubleshoot or for further clarifications, check official OnePlus forums, support pages, or trusted community resources.

If all the steps are undertaken as mentioned, you can easily flash a OnePlus 5T and get back to a stable root or stock state.