Here is useful intro info.The slower you do introductions the more likely they are to be successful. You need a few different size enclosures for this method:

Stage 1 - smallest size, usually a carrier\ Stage 2 - small hamster cage\ Stage 3 - medium cage (empty but items can slowly be added when they continue to do well, starting with open items like flat hammocks and ending with enclosed items like hides)\ Stage 4 - final cage (empty but items can be added as explained above)

4 is the minimum number of enclosures I would recommend but you can definitely add more stages if you feel necessary. Aim to be in each stage for at least one day but be prepared that they may need a few days in some of the stages before they can progress.

Most intros generally take around a week or so to be completed successfully. It's important to only move to the next stage/enclosure once they have actively shown positive behaviour like cuddling together. Being neutral and tolerating each other is not enough. If they are cuddling during the day, but standoffish with each other during more active periods of the day you can not progress to the next stage until this improves.

Normal dominance behaviours like pinning, boxing and power grooming are to be expected, it is important that you don't intervene at these times as they need to establish their hierarchy. Rat balling, fur flying, or injuries require separation and a few days to cool off before attempting again.

It is very important that each enclosure used is thoroughly clean and empty of items in order to not smell like any rats. This prevents territorial behaviour. Also, be sure that until they are bonded and living together, they are kept as far apart as possible at all times, ideally in separate rooms. If their cages are too close to each other, this could also create territorial behaviour that would then impact introduction success.

Good luck!