Check and treat her for lice and mites, worm her, and have her tested, much as you would if you were selling or shipping her. While you're waiting for the test results to come back ( keep her a few dozen feet away from everyone else, close enough to see and talk, but nowhere near enough to carry any "nasties" across.
Once your results are cleared - our Ag Ext Office takes from two to six weeks - you will have observed your new lady for long enough to know if she's carrying any respiratory or other surprises. From there, you should be good to go for putting her crate/kennel close enough to touch and socialize for a bit before you turn the girls loose together.
In the meantime, as much as it's a pain, you really do need to take care not to cross-contaminate your existing flock. If you do them first, then your new friend, just put the old clothes in the wash before you go back to your original birds. It's a nuisance, but it's a VERY smart move!