The chapter below contains a walkthrough for Mother's Heart, including screenshots and boldfaced tips.
This quest will find its way to your journal once you complete The Point of the Spear. Your first objective is to meet with Rost just before the entrance to Mother's Heart. Take care of other business or side quests for now - the quest will be waiting for you when you're ready.
Begin by asking a few questions about the Proving. Decide on how you want to react to your imminent parting - keep in mind that the choice is of secondary nature. Once you've made your choice, proceed to the settlement.
You'll be treated to a few cut-scenes. The whole tribe is in festive mood thanks to the tomorrow's Proving. Use the opportunity to visit Teb, the boy you've saved in the second mission. Talk to him to receive an outfit. You can also pick up a free treasure chest along the way.
The next step leads us to a place where Aloy meets with the representatives of other tribes. Follow to marker to find Karst. Talk to him to replenish your supplies and pick-up a free chest. Use the opportunity to engage in a short dialogue and then proceed with your new objective.
There is an another person using Focus in the crowd. Approach him and start a conversation. The objective marker will lead you to a location where the Trial participants are going to receive a blessing. Once you reach the designated area, walk up to Teersa. Now it's time to watch another cut-scene.
After the ceremony, all participants are supposed to proceed to the sleeping quarters. Use the moment to enter the crowd and talk to Olin who can tell you more about the Proving. Follow the marker to reach the hunters' lodge, a building right next to the entrance to the settlement.
Once you reach the building, talk to Bast - moreover, you can have a short conversation with Val. Finally, lie down in bed to begin the trial.