Go to Appearance.
Click on Customize.

Then click on the theme Options.

Click on Testimonial Categories.

Now choose a category you wish to display as the testimonial section, and also give a title to your description as desired. Select the number of testimonials to display.

Now click Save & Publish.

To add testimonials

On your dashboard, Go to Posts and click on Add New.

Then on the page that appears, add your desired content.

Then click on “Testimonials” to add the new category into.

To add image

Go to Set Featured Image in the bottom right of the post editor.

Then choose the image and click on Set Featured Image.

Now click on Publish to save the changes.

Now go to Appearance>>Customize on your dashboard to see the added testimonial.