Finding the witch
This quest is added automatically to your journal after completing the main quest The Nilfgaardian Connection. More specifically, after examining the chest in the basement in Heatherton village (M3,2). Go south to the Midcopse village.
Your task is to find the witch. If you don't want to waste your time on the search, you can go directly to her hut in the forest northeast from the village (M4,1). If you prefer to complete all steps, then remain in the village. You can obtain information about the location of the witch's hut in few ways. One of them is hearing the conversation of two women standing near the house, although you must stand a bit further from them and wait until they end their conversation. Another way is asking few peasants. One of them will want money for the information. You can give him the gold (50 crowns) or use Axii to temporary control his mind. After hearing the instructions leave the village. Get close to the small pond and start going across the small brook to the north. Reach a large rock at the crossroad and go east. The path will lead you to the witch's hut.
No matter how you acted, you must watch your back as there is a group of nekkers near the hut. Eliminating them shouldn't be a trouble, but if you can't deal with them you can help yourself with the Northern wind bomb.
Meeting with Keira Metz
After watching the short cutscene, enter the hut and activate witcher senses. You don't need to investigate the whole cabin, just focus on finding the interactive skull in the side room. Examining it will activate a magic portal. You must walk through it. After arriving at a new place, walk upstairs to the place where Geralt will meet the witch Keira Metz. Listen to the conversation. Once it ends, you can either immediately go with the witch to the tower of the elven mage or tell her that you will meet her at the entrance later. In both cases the quest will end (100 experience points) and you will unlock a new main quest Wandering in the Dark.