Follow the below mentioned steps to install eBay Professional Importer app to your Shopify store.
Step 1: Login to your Shopify store admin and click on the left menu: Apps.
Step 2: Now click on the button Visit Shopify App Store as highlighted in the below screenshot.
Step 3: Type the App name eBay Professional Importer in the search box at Shopify app store. Click on the eBay Professional Importer app as shown in the below screenshot.
Step 4: Click on the Get button.
Step 5: It will ask for the authorization to install the app. Click on Install app to complete installation.
Connect your eBay seller account by following the instructions at: EPI - How to link eBay Seller Account