Underlayment is something you put on top of your substrate to prepare it for tiling. A cement subfloor provides a great base for tiling. This process must be taken care of in the following steps:
Patching refers to covering small holes and gaps in the concrete. Use floor leveling compound for large holes or flaws in your cement floor. The compound is a cement-based material that is thick enough to stay where it needs to be, but also flows well to fill in depressions to ensure your floor is flat. After ensuring your concrete surface is level (the next step) apply a concrete sealer.