Unzip the downloaded release, and change the name of the folder to `fixedtextarea`.
In the `ext` directory of your phpBB board, create a new directory named `dmzx` (if it does not already exist).
Copy the `fixedtextarea` folder to `/ext/dmzx/` if done correctly, you'll have the main extension class at (your forum root)/ext/dmzx/fixedtextarea/composer.json
Navigate in the ACP to `Customise -> Manage extensions`
Look for `Fixed textarea` under the Disabled Extensions list, and click its `Enable` link.
Thanks for sharing this information with us as getting lots of error without this extension on the text area. helped me to get the solution so far and it's working fine now.