First go to reviews from main menu and click settings to view your reviews settings. If disabled, enable "Show featured reviews on Tint Wiz proposals" option:

Next, simply "PUBLISH" and "FEATURE" the customer reviews you have in Tint Wiz on reviews page to show them on proposals.

If you don't have any reviews in Tint Wiz, you can add them manually from different sources with the "+ Add New" button.