When you are praying the Rosary of the Blessed Virgin Mary, you announce the mysteries at the beginning of each mystery. It usually follows the prayers Glory Be and the Fatima Prayer (Oh my Jesus prayer) and is said before the Our Father prayer. There really is no wrong answer, as long as you are thinking (or meditating) upon them, then that is what matters. But it is always the best practice to announce them at the beginning of the Our Father and Hail Mary's.