Step 1 - In Studio, go to Settings > Member Area > Website Integration > Login Form Integration:
On this page you'll see a number of steps, ignore these for now, but you'll need some of this information later.
Step 2 - In a new tab/browser, log in to your Shopify account.
Select 'Online Store' from the left-hand menu:
Select the 'Customize' button:
Select 'Add Section':
Select 'Custom content':
Select 'Add':
Select 'Add content', then 'Custom HTML':
In this HTML box, copy/paste in the code under Step 1 in the Studio Login Form Integration:
Now you should see the 'Sign In Form' where your member can log in.
Save your changes!