What should I use to level the surface of a slab of concrete sidewalk.?

Tiling is kind of like painting a car, the outcome is based on the base prep. All your time and money will be thrown away if you don’t prepare the foundation properly; leveling products are fine for inside foundation work the foundation is usually pretty much stable. The side walk however is not stable, not stable enough to try and cover up and then tile. Concrete patches are just that, they are patches not fixes. Raise and stabilize the sidewalk sections first, then plan your tile work so the tile breaks i.e. starts and stops at each joint, if this means a little wider joint at each section then make all your joints a little wider so all looks well. Should any further settling take place re-level and redo the joints affected.