To activate maintenance mode on WordPress, we must download the WP Maintenance Mode plugin for WordPress, access the WordPress administration panel, and go to Plugins > Add new > search for the plugin and install it:

Now we go to the Plugins section, and activate it:

After activating it, we will see the website in maintenance mode with the default content of the plugin:

Now we have the website in maintenance mode, we can customize the maintenance mode by going to the plugin settings, from Plugins > Installed > and clicking on Settings in the maintenance plugin: