The new rats should be as far away as possible from your current rats, for at least 2 weeks (to avoid airborne illnesses transmitting to your current rats). Once you are sure that they have no illnesses, you can put them in cage next to your current rats, to enable them to get used to each others scents. Take care not to put them close enough that they can reach each others tails.
Once they seem to have settled down and going about their business in side by side cages, you can start introducing them for playtimes. Choose somewhere neutral, well away from the cages - like a bath tub or a bed.
They will need to establish dominance. There will be some squabbling, squeaking, wrestling, sniffing and chasing... all of this is normal. If the fights get too bad, you can use a water spray to separate them (be careful not to get water in their faces or ears). The water spray works because they have to stop to clean themselves. But, unless it gets nasty, you should try to let them resolve it themselves. The basic rule is, "no blood, no foul".
Once you have them playing happily in a neutral territory (this could be days or a couple of weeks or more, it varies per rat), you can consider putting them in a cage together.
You have some work to do when preparing the cage, too. You need to make sure that you thoroughly scrub it so there are no traces of any rats, and rearrange the furnishings so that none of the rats feel like it is their home - it has to feel like a new environment to all of the rats, to avoid nasty territorial issues. When we put ours together, we also moved the cage to a different place in the room, too.
And that's it, really. For the first few days there will be a few squeaks as they get settled in and work out who's boss. There may be some hogging of the food/water, too, until they realize there is enough for everybody, so make sure that neither food nor water can be blocked from any of them (I put an extra water bottle at the other side of the cage so they could at least get to their water at all times).
Good luck with it! Let us know how you get on and if you have any more questions.