How to make content editable from admin page?

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I think you’ll want to pay attention to two concepts:

  • Use a static page as home page, this will allow you to create a page in the backend and assign it to the front page location. You will also be able to assign a page template to it.

  • Your testimonials will likely need to be a custom post type, using the the_title() as the author name and the_content() as author description. Which you can then iterate through, and display, on a page template.