Just wait for about 25 to 30 days after she got pregnant. She gives live birth and is a livebearer, its not neccesary to have a breeder box as you dont know when to put her in and if you put her in too early she might have an early and bad drop resulting in deformities and swim bladder problems. Just put her in a bare tank with lots of plants fake or live ( Plants increase Fry Survival from Guppies as they are known to eat their own fry) so when she does give birth and your not there the fry could hide in the plants.