Simplest method to put your website in Maintenance mode, is to create a
.maintenance file in your WordPress root folder with following content.
<?php $upgrading = time(); ?>
Make sure you place this file in same folder as your
wp-load.php file. I just said that because in case you have WordPress installed in a sub-folder.
Whenever you are ready to make your website live, simply remove this file and everything will be back to normal.
.maintenance file in WordPress root will make website go in maintenance mode but it will display default maintenance message.
Briefly unavailable for scheduled maintenance. Check back in a minute.
If you want to customize this message and want to display some important information for your users, you can create a
maintenance.php file in
wp-content folder and add your custom message in it.
You can also add HTML/Image in
maintenance.php file. Go crazy.