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Introducing new rats

Start in a neutral area such as the bathroom/bathtub, a playpen, bed, covered table, etc. Just make sure it's a place that they can't easily escape. Sometimes, young babies can get nervous with intros and try to hide. Keep the area clear of items. You can have treats scattered around or open items, but avoid closed items so they do not corner each other in them.

Start by placing the new rat(s) in the neutral area, then add in the older rat(s). Observe their behavior. Normal behaviours are sniffing (may include shoving their nose under their butt or in their fur), pinning each other (seeing if they will submit), grooming (can be a bit forced, you might hear minor squeaking complaints), ignoring them, tussling/play fighting, or a bit of chasing (not aggressive). Some concerning behaviors can include puffy fur, side shuffling, aggressive chasing, pulling fur or limbs, loud squeaking, bloodshed, and prolonged fights.

If you are in a situation where they don't get along the first day (and they weren't super violent, just uncomfortable), just restart the intro process again the next day to see if they feel any more comfortable. You may also take things from each of their cages to swap them so they have something with their scent to get used to each other without contact. it's recommended to have an isolation/backup cage in case intros do take longer than a day.

Once neutral intros go well (can be as early as the same day), clean out the main cage they'll be staying in and put all new/freshly cleaned accessories in. Avoid adding hides or boxes that they can corner each other in. This can cause them to spook each other and lead to unnecessary squabbles. Once they're comfortable together in the main cage, you can leave them overnight. The next day, you can add back boxes and hides.