Sand mix is used, not mortar mix. Mortar mix contains lime and will be somewhat sticky, lime will cause the straightedge to drag. The minimum is 3/4" but striving for 1-1/4" is desired.
The sand mix is mixed very dry then packed (NOT POURED) into place. The straightedge (
"screed") is dragged over the surface to achieve the plane. Steel pipes can be used to establish level and to drag the straightedge on top of. Once the surface is flat the pipes are removed and the voids filled and scraped flat to match the surface.
A surface primer is required. Liquid concrete primer will work. Thinset tile mortar can also be used either thrown down dry or mixed and trowelled to bond the sand mix to the existing concrete.