Leave food, water, and cat litter in the room. The mother won't want to leave her kittens for long in the first two weeks of their life. Always put food and water within stretching distance of her nest, and if possible, offer a litter tray in the same room so that she can stay within sight and sound of the kittens.
If food is in another room, some mothers chose to starve rather than leave their newborn kittens to find it.