Log in to your Shopfiy store, then open a new browser window and go to Sandboxcommerce.com and click 'Get Started for FREE'
From the dropdown menu, select 'Shopify'
Click 'Add Unlisted Sales Channel'
Create an account password for your 7 day trial
Select your subscription plan (This will take effect after the trial period has ended, if you choose not to cancel)
Click 'Start Free Trial'
That's it! You're now connected to the Sandbox Commerce app builder!