Curing is the process to keep the internal moisture up which continues the hydration (chemical reaction of water and cement is known as hydration of cement).
While there are shuttering (for slab and beam 21 days generally), the top surface is waterlogged and other surfaces are moisture-proof by shuttering material. So there is no deficiency in curing. After shuttering has been removed, the bottom surface may face some moisture loss. Generally, extra action not taken to counter this and it has not a major effect on strength of the structure.
For open beam, you can cover it with Hessian cloth (Juice Bags) and sprinkle water over it. For Slab, you can sprinkle water on the bottom slab.
Application of curing compound is also a method but it is costly.
For curing of slab in winter, I suggest changing the water daily as it helps to keep the water temperature and continue curing up to 28days.