While connected to Wi-Fi, plug your USB Flash Drive into one of the USB slots on the Chromebook
A notification will appear in the bottom right corner
Click on Open Files App
A new Window will open
Click on Google Drive
Click & place checkmarks beside the files you want to save
Click & drag your file to your USB Flash Drive
Or use ctrl+c and ctrl+v to copy & paste selected files to the USB Flash Drive
You can access the files on your USB Flash Drive for offline use on your Chromebook. You will have to upload these files back to your Google Drive if changes have been made to them.