Domain Mapping and WordPress Multisite

Hello,

I am trying to utilize WordPress multisite to use with my client’s who each have their own WordPress website under my cPanel. However, instead of having multiple WordPress installs, I would like to merge all of these into one install using the Multisite feature. I understand the security risks with this so we do not need to touch on that but I am having trouble setting up the domain mapping. I have read the Codex and all other relevant documentation but nothing has been helpful so far. In other words, for example, I want to have abc.com/123 point to 123.com. Can someone please provide with step by step instructions as to how I can accomplish this? Please be as specific and detailed as possible as other guides have not been helpful.