This page of the guide to The Witcher 3 Wild Hunt contains a walkthrough of the main quest - Echoes of the Past. The following walkthrough explains how to help Yennefer defeat the foglers, how to use the mask received from Yennefer, and how to find the tree related to the quest.
After completing the first main quest The King is Dead - Long Live the King, go East from the druids' camp, where you will meet Yennefer and Mousesack. Join the conversation and then follow the druid to the forest. There you will find Yennefer fighting muckixers (level 17). Help her out. Once you get rid of enemies, put on the mask received from the sorceress (new quest item, used like a crossbow).
During the rest of the quest you must follow Yen through various places where, when she tells you so, you put on the mask and watch the trails of past. Between successive points you will face muckixers on the way.
Last point is the fallen tree stacked into the ground. Look at it with the witcher senses and the sorceress will move it. After the cutscene you can decide whether you want to continue the quest immediately, thus moving with Yennefer to the next point, or do you want to get to Loften village on the eastern island by your own. Next quest is named Missing Person.