All surfaces should be thoroughly cleaned of all dirt and debris. When installing over an existing concrete base, it is important to remove all traces of existing finishes, contamination, etc. to allow for a clean surface.
For thicker pour of up to 6”, elevate the wire mesh on which the cable is tied with bricks, wood blocks or other spacers to float the cable at the desired level. Depending on the construction of the subfloor and in general, in order to improve the efficiency of the system, you may need to apply insulation prior to the pour.