A:A lot depends on how flat the surface is to start with. If its quite smooth and level give it a coat of builders adhesive first and leave 24hrs (read the instructions on can for sealing serfaces) then tile as normal, I prefure a waterproof combined adhesive / grout.Over 2 days
If the surface is uneven do the builders adhesive/cement skim/builders adhesive/Tile route over 3 days