Depending on the size of the crack, use a putty knife or trowel to apply a vinyl patching compound in multiple layers as directed by the manufacturer. Mix only the amount of patching you'll need for each layer. Press it firmly to force the initial layer into all nooks and crannies and allow drying time between layers.
You can also patch thin cracks with a cement mixture. In a small bucket, mix one part cement and three parts sand with enough water to make a stiff paste. In a separate container, mix a small amount of cement with more water — enough to make a cement "paint."
If you can, ''paint'' the inside of the crack with the thinner cement mixture. This will act like a primer. Then take the paste mixture and pack it firmly into the crack with a putty knife or trowel. Level the mixture with a straight-edged concrete-finishing trowel. Let the patch sit undisturbed for about an hour, then go over the entire surface with a float or trowel in a circular motion, blending it well with the surrounding surface.
When applying the patch, make sure you cut deeply into the mixture with your trowel to remove any air pockets.