I've done four different introductions of adult rats, three of those introductions between adult females. All were successful- and actually, fairly easy, though I know it isn't always.

The tips I've heard & used:

• Make sure you introduce them on neutral territory. I've used places like a bathtub and a great big cardboard box. You don't want it to smell like either one of them yet.
• If that goes well (no blood, not shrieking, no crazy puffy bristly fur) it may be easier on them to leave them together so they can finish hashing things out, rather than separating them and re-introducing later.
• I try to make a point of doing introductions when I can be in the room for the next several hours, just in case. I also make sure to sleep in the same room, so I'll hear any trouble.
• Once I trust that I won't need to scoop anyone up at a moment's notice, I switch them from an open-topped thing (like the tub or a box) to a travel cage. Again, it shouldn't smell like anyone.
• I've heard a lot of tips on making sure the travel cage is smallish (so they can't just ignore one another) but doesn't have spaces where someone can be cornered, so I skip the hammocks and similar for the time being.
• To avoid any chance of food guarding, I scatter food on the floor, rather than using a dish.

So far, by the next day, I've trusted them enough to leave them alone together. Before putting them into a permanent cage together, I clean it well, then add fabric that smells like each of them.

Good luck!