I haven't done this in quite a while. So, like Jake stated, you'll probably want to reference the documentation.
That having been said, it's really a two-step process. First, you need to add the field to the database. Second, you need to add the field to the form. There are all kinds of things that you might want to take into consideration...but, we'll just leave this at the higher level.
It's not terribly difficult. Check to see if there's someone on your team that has done this in the past and can guide you. After the first couple of times, it becomes second nature.
If you have specific questions about where you're having issues, post those questions. Really, the first couple of steps are up to you. While we're at it, you might want to look to see if IBM/Rational offers any CQ-related training in your area.
"The single biggest problem in communication is the illusion that it has taken place."
-George Bernard Shaw, Irish playwright and Nobel Prize winner, 1856-1950