Unzip the downloaded release, and change the name of the folder to `totalattfilesize`.
In the `ext` directory of your phpBB board, create a new directory named `dmzx` (if it does not already exist).
Copy the `totalattfilesize` folder to `/ext/dmzx/` if done correctly, you'll have the main extension class at (your forum root)/ext/dmzx/totalattfilesize/composer.json
Navigate in the ACP to `Customise -> Manage extensions`
Look for `Total Attachments Filesize` under the Disabled Extensions list, and click its `Enable` link.
dmzx/Total-Attachments-Filesize
