There are two ways to delete a custom page:
Navigate to Publish > Pages
Find the page you want and select Edit
Scroll down to the very bottom of the page, and select the red Move to Trash button on the bottom left corner of the Publish section
Alternatively, from the Publish > Pages screen, once you find the page you'd like to trash, you can hover over the page title and select Trash without clicking into the edit screen for the page.