After marking out with masking tape, I then tap the surface with a centre punch. This breaks the glazing and makes it easier for the first drill size. Fresh bits are always best, but I have been using the cheap craftright brand from bunners with fairly good results. I drill with no hammer function to get started. Always drill last hole size slowly for the plug to grip. Bit of 30 min. polyethylane foaming glue never goes astray. Porcelain tiles are a pain.