I introduced my new rats to my old rat in a jacuzzi bathtub that had some towels and boxes in it. I figured having places to hide would help them be calmer. After it went well, I then let them run around on the floor of the bathroom. This site helped me a lot to be prepared:
http://www.fancy-rats.co.uk/information/guides.php?subject=introductionsI would also pick up a cheapo spray bottle from Target or something and fill it with water to spray at the rats if something goes bad. They usually get startled and wander off to clean themselves.
I will say the whole process was a bit weird. At first, the rats just said a brief "hi" and walked away. After a longer period of time, they started trying to dominate each other, the older one trying to box and kick at the new duo if they got to much in her face.
This is also something you should do over a period of days before actually putting them together in a (completely cleaned) cage. Once you put them in a cage, even that should be stages, not leaving them immediately in there without supervision.
Hope that helps!