By an international collaboration the KArlsruhe TRItium Neutrino experiment KATRIN is currently being installed and commissioned at Karlsruhe Institute of Technology (KIT), the site selection that makes sure of the unique expertise and infrastructure of Tritium Laboratory Karlsruhe (TLK). KATRIN requires a strong windowless gaseous source of almost pure molecular tritium (95%) and a throughput of 40 g tritium (1.5·1016 Bq) per day, stabilized to the 0.1% level. Since the last large components have been delivered in summer 2015, the collaboration is now focusing on the commissioning of the whole KATRIN experiment. A particular challenge is the commissioning with tritium, which will mark the point of no return regarding the contamination of the large magnet cryostats and tritium loop components. We have developed a 5-phase plan that covers all necessary work to be done for the safe and reliable standard tritium operation of KATRIN.