First and foremost you need to do a 30 day quarantine. This means the new chins need to be in a completely separate room from your current chinchilla, no shared playtimes, washing between handling, changing clothes between rooms, etc. It takes 30 days on average for contagious diseases to present themselves in chinchillas. Even if the chins are coming from a reputable source, it is best to do the 30 days.
After that there are numerous methods of introduction, but you won't need to worry about any of that for a month.