I keep the Martyrs divider tab in the back of the book until I need it.
A SEASON or SAINT'S day will sometimes direct you to read parts from the Martyrs Feasts P. 1250-1271. In those cases, I place Martyrs divider at the divine office being prayed (P. 1250-1271).
If you are directed to read from the Martyrs section then the Martyrs parts of the divine office replace and supersede the same parts in Ordinary/Weekly Psalter pages 903 - 1141.