Before laying floor tiles on a concrete subfloor, it is imperative that you make sure that the floor is level. Once you have removed any bumps and raises with a cold chisel or an angle grinder and diamond blade, then you must fill in any dips, gouges, and cracks with floor leveling compound. If there are only a few here and there on your floor, then that's a fairly simple filling job. If your floor however is pitted in numerous locations, then it may be best to do the whole floor with a mix of the appropriate self-leveling floor compound in order to give it a complete overhaul.
As a general rule of filling dips, any gouges that are found to be deeper than 1/4" inch or wider than 1" square inch should be filled and leveled before applying your bed of floor tile mortar. Anything less than these sizes will generally fill naturally when the mortar bed is applied. These can easily be found by sliding a spirit level or any other long flat surface across your floor, or simply just with a keen pair of eyes and good sense of judgment.
4 1 2 Inch Grinder
Before filling with your floor leveling compound, first brush out any dust that is resting in the dips. The use of a cheap paint brush is ideal for when dealing with fine dust, but a regular dust pan and brush will do almost the same job. Next, wipe the gouge with a wet cloth to activate the existing cement in the concrete. This aids in giving the leveling compound a solid bond with which to adhere to when applied to ensure a firm fixture of the repair patch.
Follow the instructions carefully to whichever floor leveling product you are going to use so to get the correct consistency of mix, then apply to the dip, level with the flat edge of your plain floating trowel or your notched trowel, and then wait at least twelve hours until dry before applying mortar and laying floor tiles.
When dealing with narrow cracks in the concrete floor, the same method of leveling should apply, but before doing so you should actually make the crack wider to ensure a positive fill. By taking your cold chisel, hammer, and safety glasses of course, simply chip away at the concrete surrounding the full length of the crack to at least make it a minimum of 1/8" inch wide, and then follow the same procedure as mentioned above.