First, create a Shopify Social Proof app on Common Ninja.
On the apps catalog, find the Social Proof and enter the Editor.
When you're done creating the Social Proof app, click on the Save Changes button at the right top corner of the editor.
If you don't have an account on Common Ninja, you'll have to register first.
When you're ready to add the Social Proof app to your Shopify store, click the Add To Website button at the top bar of the Editor.
On the window that opens, click the Copy Icon to copy the app embed code. In the next steps, you will paste this code into your Shopify store.
Head to your Shopify admin panel, and on the side panel, enter Online Store.
Click on Customize Them.
You can select the page to add the Social Proof app.
Add a new block or Section to your page, wherever you wish the Social Proof app to show.
You can do it in part of Shopify's themes by clicking on the "+" button showing between sections.
Or you can add a new section on the side panel and then drag and order it to show where wish on the page.
Select the Custom Liquid section type.
Click on the new section that was added to your page.
Now, simply paste the Social Proof embed code you've copied before in the Custom Liquid Code Windows on the side panel.
Click Save at the top right corner of the editor to save your changes.