You need 4x Steel and 5x Cloth to build each bed.
Choose one of the Beds and place them in your settlement, in an area that has a roof such as one of the houses. I've agreed to help the people in Sanctuary establish a permanent settlement. This quest objective is part of the mission helping Preston Garvey build a safe settlement for his men. Build sheltered beds for Sanctuary settlers is a quest objective in the mission Sanctuary in Fallout 4.
