Login to your current domain registrar and obtain an "EPP" or "Auth Code" or "Transfer Code" from them for a domain transfer.
Important: You may also have to unlock the account so it can be transferred.
Make note of the Name Servers, you’ll need them when you register on this side.
Login to the Control Panel
Select Billing -> Transfer Domain to Us
Enter your domain name and click Check Availability
From the resulting list go down to the bottom and click Add To Cart
Enter the EPP code you got from your current domain registrar
Decide whether you want ID Protection: this will replace your domain registration contact information with generic information so that your identity is protected.
Change the Name Servers to the ones that showed on your old domain registrar
Click Update Cart
On the Domain Order Summary screen, enter any Promotional Code you have for discounts and click Validate Code
When ready, click Checkout
Note: there may be a waiting period of up to 7 days before the actual transfer takes place