How do you fit an aldi simply bathroom tap so the tap does not simply lift off?

There is a tool and two 'c shaped' washers sticky taped to the outside of the polystyrene box. Inside the tap head is a brass collar which you'll need to unscrew out. Put the collar on the tap shaft, the c shaped washer in the groove on the shaft, put the tap head on and then secure it by screwing the collar into the tap head with the tool provided.