How to repair masonry porch riser

The loose material you are wanting to repair is called "parging". It may be referred to as a skim coat as well. It typically is a coat of mortar, not cement. Mortar has more limestone in it to make it more workable.

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You can easily remove the bulk of the loose material by tapping the surface with a hammer. It should readily crack and fall off. to clean up the areas that meet the top of the landing will need a little help, since the parging will not necessarily crack cleanly at the top of the slab. That is where a chisel can come in handy. It does not need to be any specific size just something that has a straight cutting edge on it. Even an old crappy screwdriver will work in a pinch.

After everything that was loose has been removed, clean the surface with water and a stiff bristle brush to remove all contaminates. Let it dry a little. then start the applying of new material after the areas are protected from excess mortar. If you do the new application the next day, the surface will be still damp enough to help aid the the curing of the new coat.

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Using the proper sized trowels and surface protection, apply the mortar to the wall in a thin layer about 3/8" thick, let it set up firmly then with a fine haired brush, like a soft shop brush sweep the surface with it to give the sanded finish.

Typically the finish you see is for esthetics only, not for weathering, but to be safe, use type "S" masonry cement. It is what is used for waterproofing the outside of basement walls in CMU (Cement Masonry Unit) construction.