The trick to freezing berries of any kind is to allow the berries to dry thoroughly before freezing. The water is what causes ice crystals to form when freezing so the more you can allow the berries to dry the better. With that being said you do not want your berries to sit out at room temperature for too long or your berries will continue to ripen. Your best bet is to do this quickly so you are freezing the freshest berries possible.