The secret is one leg at a time.
Lower the trousers and push one trouser-leg down inside the shackle, between it and the leg, until you have stuffed through as much as will go, including some of the crutch. Then pull the shackle as high up the leg as it will go and remove your foot from the trouser-leg which should now all be below the shackle. Next feed back all the loose cloth of the trouser-leg back through the shackle. One leg is now free. Repeat the process on the other leg.
Putting them on again merely invokes reversing the operation.
J.M. Kariuki, Mau Mau Detainee