You can connect your Shopify account two different ways:
Option 1 - Adding Partial.ly from the Shopify app store
Option 2 - Connecting Shopify from your Partial.ly account
Both will work just as well. Option 2 will require you to sign up for Partial.ly first.
Option 1: Go to the Shopify App Store and find the Partial.ly app. Click the "Get" button on the right side of the page. You'll then need to login with your Shopify credentials and authorize Partial.ly.
Option 2: Sign up for a Partial.ly account at https://partial.ly. Go to the integrations settings page of your Partial.ly merchant account. In the Shopify section, click the connect my Shopify. You'll be taken to a Shopify authorization page to authorize Partial.ly.