Thanks Chris. The reason I was asking is because my daughter recently did this but she mustn't have prepared the concrete because in time, little bumps are showing and little dents.I wouldn't be able to prepare the concrete as described and wondered if I could put some type of underlay down like hardboard or similar before putting the tiles down.
You need to have the floor flat, dust free and level before starting or you get the problems you mention. Get the floor as flat as possible first then use a leveling compound to fill any holes or dips. Allow this to dry completely, sand any rough bits and remove the dust then either use self adhesive tiles or ordinary ones that you apply adhesive to.