It depends on your fish, of course. First, make sure your girl tank is large enough for additional fish; if your resident girls feel they are overcrowded with an additional inmate, they will not take kindly to her no matter what you do. If the tank is big enough, what we do is put the new girl in a clear tank/container and "float" it inside the big tank so everyone can look at each other, get used to seeing each other--for a week or two (no shortcuts). What we mean by "float" is, we have these large "Specimen containers" made of clear plastic that hold about 1/3 gallon of water, they have this plastic hook-like affair at the top edge that goes over the side of the tank so there's no chance of it submerging. Only fill the container no higher than 1/2" from the top (less of a chance the new girl will jump out). Clean the smaller container daily because the waste will build up fast and you don't want her getting sick. Usually, the resident girls will get all excited and try to get at her but after a day or two, they get used to the sight of her and come to ignor her. By the end of a week (2 weeks of one fish seems to continue to try to attack new girl), usually everyone is so used to seeing each other that you can release the new fish in with the other girls. There'll still be a bit of pecking order being established once she's among the rest but it should settle down fast. This isolation will also give you a chance to make sure the new girl hasn't brought any fungus/bacteria/parasites with her and gives her a chance to get used to new surroundings.