firstly, you can inject breast cancer cells intramuscular in the leg of a nude mice. I suggest before starting to check that the cell line that you want to use is tumorigenic in nude mice e.g. you can inject a different numer of cells and check the efficiency to form tumors.When you have try the right number of cells to inject, you can start your experiments. generally, when the tumor became palpablity you should start your treatment.