Copy the extension to phpBB/ext/phpbb/skeleton
Go to "ACP" > "Customise" > "Extensions" and enable the "phpBB Skeleton Extension" extension.
In order to create an extension via the web UI just open your board an visit the "Create skeleton extension" link at the top left:
https://localhost/phpBB/app.php/skeleton
The .zip is then offered as a download. Additionally it can be found at
store/tmp-ext/.
In order to create an extension, you need to open the console of your server. Then run the following command in your phpBB root (next to config.php):
./bin/phpbbcli.php extension:create
Afterwards copy your extension from store/tmp-ext/ into the ext/ folder