Setting up a GoDaddy SubDomain and Connecting to Shopify
GODADDY SIDE
Log in to GoDaddy and navigate to the Domain and click the 3 dot ellipse (…) > DNS Management 
Under the records section click ADD.
Change the settings to the following:
Type: CName
Host: shop (or whatever you want the sub-domain to be).
Points to: your Shopify URL (for EG: third-day.myshopify.com)
SAVE 
SHOPIFY SIDE
Login to your Shopify Store > Online Store > Domains
Click Connect Existing Domain
Add the domain name created in GoDaddy.
For example: shop.thirdday.com
Click Verify