How to restore a flaking / peeling concrete step before painting?

MichaelMoneySaver

Hi, please could I have some advice? I'm restoring my elderly mum's front door and step. The area is in a bit of a state of disrepair. There are quite a lot of things needing done to her house so I tackle them myself and learn along the way whilst aiming to save her the cost and interaction with tradesmen.

I would like to apply red masonry paint to the step. But how do I prepare the step first? The step appears to be concrete, distinct from the red sandstone surround.

You can see that the actual concrete forming the vertical back wall of the step has peeled / flaked away at the left side where the most footfall is.

How can I level the back of the step to the same depth or alternatively restore the peeled concrete by adding some kind of filler so that the step is smooth and ready for painting?

(In the photos, the draught excluder from the bottom of the door has been removed, exposing unpainted wood - this area is being sanded and painted separately)

Thank you in advance!