In the past we did a lot of granolithic skirtings (internally), made from a mixture of sand/grano dust, and cement, very similar to your job. It was put on in 2 coats, a scratch coat one day, and a top coat another day. A half inch thick batten was fixed horizontaly to the wall first, at the required finished height of the skirting. A scratch coat was then trowel applied, (out to "half the thickness" of the batten), scratched, and then left left to set and cure, (day or two max). The top coat was then put on. It was done similar to cement render. Bring the "top coat out flush" to the thickness of the batten. Once the top coat is on, it is left to pick up, and rubbed up with a float. Make sure the face of the cement skirting is vertical, don't play with it or it will slump. You can either leave it finished off of the float or a sponge, or gently pass a trowel over it to smooth it off. Leave the top batten fixed to the wall for a day or two, (at least) until the skirting has set and cured. If you try to take the batten at the top off to soon, you will badly chip/damage the top edge of the green/newly formed render skirting. Once the wall batten has been carefully removed, you can then neatly dress/sponge along the top of you newly formed skirting with a small amount of render mix. You can also "carefully" rub a fine, wet carborundum stone along the top edge corner, to take away the sharp edge of the skirting, and make it a neat, slightly "pencil round" top edge. Most of the skirting we did also had a cement/grano cove at the bottom, blended/feathered into the floor screed. Your's is only a very small repair job, but this "over long" reply gives you some idea of how it's done.
D.