For the first week, keep the cats separated. The new cat should stay in one room with her own food, water and litter box.
On the eighth day, place the two food dishes on either side of the door to the new cat's room so the cats will learn to associate each other's scent with a pleasant activity. Move the bowls closer with each feeding, reaching a point where you can briefly crack the door open so the cats can see each other while they eat.
When the new cat appears to be comfortable, place the current cat in a closed room and let your new cat explore the apartment.
Now it's time to let the two pets meet. Some hissing and wariness is normal, but if their behavior is too aggressive, repeat the initial process.