After completing A New Path, you will find the quest in your Journal.
Travel to Merrill's Home and talk with her about what happened.
NOTE: If you want to get the Romantic achievement with Merrill during The Last Straw, you have to choose the option "Stay with me" at the end of this dialogue. This option is only available if you previously begun a romance with Merrill (flirted with her, asked her to stay at your house, etc).
Ensures her loyalty for The Last Straw.
If you try to change your party, Merrill will be locked on and you can only choose 2 other companions. If you travel back to the Hawke Estate and re-exit the problem is solved.
This scene is supposed to play after this quest. It is erroneously set to trigger when it is Act 3 and Merrill Relationship is Locked (ie, full friend or full rival). It does not check to see if the quest is actually completed. This causes the dialog to show up at the beginning of the act for those who quickly went to extremes with Merrill or romanced her. This can also cause the scene to fail to show if the relationship remains neutral. If the scene shows early, it may be necessary to talk with Fereldan's ruler, return to the estate, or use the console to restart the quest. On the PC, this can be fixed via save game editing. (This seems to have been fixed in PC patch 1.02/console update 1.01.)