Empty out the fountain of any water if the crack is on the inside.
Mix the concrete paste or 'grout'. The amount of grout you make is dependent on how much you need, however, when mixing the cement, maintain the ratio of 3 parts sand, 1.5 parts water and 1 part cement.
Moisten the area around the crack you are about to fill. This prevents the grout from being dried out after applied, however, make sure the area isn't too wet.
Apply the grout with a putty knife or pointing trowel and try and get as much of the grout inside the crack as much as possible.
Cover the area of the crack with a piece of plastic for protection while the grout is drying. This should take around five days.