Creating add-on domain at Bluehost

Step 1: Log into your Bluehost Control Panel.

 Step 2: Click the Addon Domains icon in the Domain Management section.

Step 3:  Enter your domain name without "www" in front of it. You need to verify ownership if this domain is not associated with your account.  After verifying, click assign this domain.

Here's a video from Youtube, how to create Addon Domain in bluehost.