We already started saying it in the beginning of December. This essentially means that the shemona esrei’s that you said was not valid, and the next tefilla that you daven you should daven shemona esrei twice 9 called a tefils tashlumin). My assumption is that you will see this in the morning, so you will be making up the maariv shemona esrei at shacharis. You will say the second shemona esrei after saying ashrei of ashrei- uva ltzion. Then say shemona esrei again as the makeup one, and then continue with lamnatzeiach.and u’va l’tzion.