The concrete must be fully cured.  Once you choose your stain, you want to protect all areas that the you don't want the stain to touch (walls, baseboards, cabinets).  Make sure the floor is free of dirt and debris and competely dry.  Then apply the stain according to the manufacturer's recommendations.  If you want deeper colors you should apply it a little bit heavier and if you want soft colors, apply less.  Most stains require two coats and then you have to scrub off the residue once dry.  Once that is complete you will want to use a protective sealer.