Reassembe blade shaft together in same order, taking note that the bladed are in correct orientated (it is possible to put them upside down by mistake).

Clamp the head portion in a vice (with cloth) and tighten with adjustable spanner in an anticlockwise direction (left-handed thread)until tight. Do not overtighten as you risk stripping the base. The assembly should self tighten once back in the thermomix anyway.