Log into your account and find your domain settings page located in the drop-down menu. You can also click this link.
Once there, select Domain Settings. In this section, you'll be able to authenticate a domain to improve email deliverability or update your sender profiles. Click create profile to make a new sender profile.
Follow the instructions and set up your new profile, like below. Click "Create Profile" to save it. To edit a profile, simply click "Edit" next to the name.