Bluehost
From Shopify Wiki
Login at Bluehost
goto cPanel
Scroll down to Simple DNS Zone Editor
Add an CNAME Record
Name "www" or "shop" etc
CNAME "dropify.myshopify.com"
Click "add CNAME record"
Done!
PS: Replace dropify with your myshopify subdomain, of course!!
If you run into any trouble, you will find it more helpful to first contact Bluehost for support as they have access to those settings (while Shopify doesn't). Here is a direct link to their support page.