Step 1: Create a new page by going Pages >> Add New
Step 2: Name the page and set Template in Page Attributes to Coming Soon or Maintenance
Step 3: Publish page
Step 4: Navigate to Appearance >> Customize >> Maintenance Mode >> General
Change Page to the page you have created already.
Step 5: Do not forget to hit Publish
Notice: You must log out to see Maintenance Page
Step 1: Go to Appearance >> Customize >> Maintenance Mode
Step 2: Scroll down to customize Coming Soon or Maintenance section.
Step 3: You can customize Title, Text, Countdown and Mailchimp Form here.