The video will walk you through the process.
First, go to your current registrar where your domain is currently hosted (ex. GoDaddy) and make sure your domain is unlocked. Then get a transfer/authorization code for the domain. If you don't see an option to get this code in an easy to find location, you can contact their support for assistance.
When you have that code:
If you DO NOT have an existing domain.com account, you can go to this page below and follow the steps to transfer the domain!
https://www.domain.com/domains/transfer
If you DO have an existing domain.com account, simply login and click on the blue button in the top right corner that says "Transfer a Domain".
Throughout the process of transferring your domain keep a look out for emails from both your old registrar and domain.com as there may be some confirmations to make.