From a laptop, sign in to your iCloud account here: http://beta.icloud.com
Click on Account Settings >
Scroll down to Custom Email Domain and click Manage.
A pop-up will appear, click Add a domain you own.
Select Only You. You can also select You and Your Family if you have family associated with your iCloud account and want them to have a custom email too.
Input your custom domain (example: yourcustomdomain.com). Click Continue.
In Step 2 click Skip (for now).
Go to Step 3 and click on View.
A pop-up will appear with the title Update your domain's records. Keep this tab open for now.
In a new tab, go to godaddy.com and sign into your account.
In the top right corner of the site you'll see your first name, click on it. A drop down will appear.
Click My Products.
Scroll down to you see your domain name under All Products and Services. To the right of your domain name, click DNS.