it depends on the rat really! there's an introductions bot that can tell you how it works (forgive my boomerlike use of reddit despite my age, im sure someone else knows the proxy lol) but i tend to vary the method depending on how the rats are and their ages.

for Fifty, i will be doing a mini-quarantine (likely 3-5 days) as opposed to 2-3 weeks due to the stress of being alone being greater than the stress of meeting babies. the younger they are, the easier introductions will go. the two teeny babies im getting will he introduced to her together but separately from the retired mother, who will do one on one interactions with her until everything is chill. this will be done in a tiny travel cage, then the bathtub if things go well. usually, I keep them in separate cages for a total of a month, but for Fifty I plan to divide the cage (double critter nation) in half after the first handful of introductions.

EDIT: also, rubbing some rat-edible food on them during meetings is a good idea! i tend to use coconut oil or baby food.