You can easily enable maintenance mode for your WordPress website using a plugin WP Maintenance Mode, which will provide you with some customization options. To install it, access your WordPress Admin Dashboard (usually, it’s available via domain.tld/wp-admin), hover the pointer over Plugins, then click on Add new.
Once there, search for a plugin called WP Maintenance Mode, install it, and activate it.
After the activation, configure the plugin’s settings and turn it on. To do this, go to your left sidebar and choose Settings → WP Maintenance Mode:
Fill in the details under the General tab. If you want to leave the default design, you can enable the maintenance mode right away.
If you would like to customize the page that visitors will be seeing (add the background image, colors, etc.), you can do this at the Design tab.
Additional Resources
You can find a comprehensive guide about Maintenance mode, together with additional ways to enable it, in this tutorial: How to Enable WordPress Maintenance Mode.