Log in to your Shopify store, and in the bar on the left side in the "Sales Channels" section, click on the ⊕ icon.
A popup will appear, and by scrolling to the bottom of the list, you'll find a section called "Don't see the channel you're interested in? Visit the Shopify App Store". Click on "Visit the Shopify App Store" to be directed to the Sales Channels section of the Shopify store.
In the search bar at the top of the page, type "Shoppy" and enter the search. In the list of results you will see Shoppy - Mobile App Builder, and by clicking on it, you will be taken to the application page.
From here, you can simply click on the "Add App" button to install the app. Once the app is installed, it will be located in the Sales Channels section, in the left bar on the Dashboard of your store.