Assess and prepare
The first step is a proper look at the slab. The concreter checks whether it is structurally sound, whether the cracks are stable hairlines or active movement, and how well an overlay will bond to what is there. On Bendigo's reactive clay they will pay attention to any signs the slab has moved or lifted, because a surface that is following ground movement needs different treatment to one that is just dirty and faded.
Prep is most of the job. The surface is cleaned back, usually by grinding or high-pressure cleaning, to remove old sealers, coatings, oil and loose material so the overlay can grip. Stable cracks are dealt with, and control joints in the old slab are honoured in the new surface so they do not telegraph through and crack the overlay in the wrong spot.