Hi Matthew. I'm Greg, 10 years awarded Windows MVP, here to help you.
First Clean and format it using this guide:
http://www.howtogeek.com/235824/how-to-clean-a-...
Then Follow the steps toward the bottom of the download page here in the section "Using the tool to create installation media:"
https://www.microsoft.com/en-US/software-downlo...
Make sure it completes as it is a multi-step process that downloads, writes the ISO to the stick, and then makes it bootable.
If it fails after the above then try another flash stick. There's a good one here:
https://www.amazon.com/SanDisk-Ultra-Transfer-S...
Once it's successfully formatted and confirmed to boot properly, you can add other files as long as you don't touch the seven files and folders the tool places in the stick.
Report back results for each step so I know what else to suggest.
Feel free to ask back any questions. Based on the results you post back I may have other suggestions if necessary.
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