Use a sturdy object, such as a brick or piece of wood, to keep the blade stable.

If using a brick or similar object, place it on the right side of the blade because we'll be turning the socket wrench to the left.

If using a piece of wood, place it across the bottom of the mower, so that the right side of the wood is wedged against the blade.

A small piece of wood is the most common way to stabilize the blade.