Non-Section Themes:
The Shopify Product Reviews app user interface can't be installed in narrow columns, such as directly under the "add to cart" button or product description on Out of the Sandbox themes.
Because of this, we recommend installing the app interface below your image gallery and product description columns so that it can span the entire width of the page container.
To add the app's code, open product.liquid and then look for this code:
{% if settings.display_related_products %}
and paste the code provided by the Shopify Product Reviews app just above this.
Please note that this only applies to the Shopify Product Reviews app. Other review apps may have different requirements or installation steps. For assistance with these apps, please contact the app developer.
Section themes:
The Shopify Products Reviews app code is already implemented into the theme's code. In order to enable product reviews you must:
1. Download the app: https://apps.shopify.com/product-reviews
2. Open the Theme Editor > Theme Settings Tab > Product grid > Reviews