How do I put a Page behind a Login screen?

A login screen is presented based on whether or not the page is designated as 'Customers Only'. You can see this option when editing a page.

Head to Content.

Then click on View pages in the Pages section.

Select the page you wish to put behind a login screen.

In the top right you will see the tickboxes for 'Published' and 'Customers Only'. Select the customers only box.

Finally press save in the top right corner.