With an assistant:
The assistant stands on the medial side of the injured leg and maintains some flexion of the knee and the right angle position of the ankle, pressing gently on the knee and holding the big toe forwards in line with the patella.
The foot is also held in such a manner as to tend to pronate the foot.
This usually holds the reduction that has been achieved and allows the operator to apply the padding and the plaster cast.
Without an assistant:
Apply the plaster padding
Then, apply around the ankle one roll of POP (dipped in cold water to give more time until hardening), reduce the fracture and maintain it reduced until the POP skim is hard enough to hold the reduction.
Then complete the cast.