1. Before you head over to Shopify, ensure you've gotten your API Login ID and Transaction Key using our easy-to-follow Authorize.net Startup Guide. You may want to leave Test Mode on and only turn on Production Mode 
2. Visit the Shopify App store and log into Shopify if you are not already, log in using your Shopify username and password.
3. Click Install App
4. Click Connect to Authorize.net
5. Enter your API Login ID and Transaction Key.
6. Click Connect to Authorize.net
7. Choose Settings --> Payments
8. Select the card brands that you accept.
9. Submit a test order through Shopify. If you are still in Test Mode, now is the time to switch to Production Mode. 
10. Click Payments. Authorize.net should now be listed as a supported payment method.