After logging in to your yahoo small business account, select to create a forward.
Create a forward for yourdomain.com, www.yourdomain.com to yourshopname.myshopify.com
So if the name of the domain you wish to use is williams.com and your shopname is williamsshop.myshopify.com, you would create a forward from williams.com, www.williams.com to williamsshop.com.
Next go to Advanced DNS Settings
Edit the CNAME Record *.yourdomain.com and change the destination to yourshopname.myshopify.com.
Next, go to your shopify admin page.
Click on the link "I would like to claim another Hostname"
Then make it your primary domain and check the box before "always redirect customers here"
Then claim yourdomain.com (i.e without the www)
And thats it