First you will need to access your website editor. If you don't know how to do so, you can read this article
Once you are in the website editor, on the left hand side bar there is a pin labelled "Testimonials" that if you hover over a menu will open up and you will select "Add New Testimonial"
You will now be taken to the page below where you fill out the testimonial information
One you have finished entering the testimonial information, on the right side of the page you will press the "Publish" button to finalize the testimonial.