Step 1: Create a new page by going Pages >> Add New
Step 2: Name the page and set Template in Page Attributes to Maintenance
Step 3: Publish page
Step 4: Navigate to Appearance >> Customize >> Maintenance >> General
Turn on the Maintenance Mode then change Page to the page you have created already.
Step 5: Do not forget to hit Publish
Notice: You must log out to see the Maintenance Page
Step 1: Go to Appearance >> Customize >> Maintenance
Step 2: Click on Maintenance
Step 3: You can customize the Background, Logo, Title, Countdown and Mailchimp Form here.