Very simple and quick to do. The two arms of the frame each have a welded-on, upstanding pin which is placed in front of the leaf-spring as widely apart as possible. (On other cars they would be against some other static part of the frame or suspension).
On each side, a heavy-duty chain which is welded to the pin is guided over the back of the spring, pulled tight and then the hook of a ratchet-strap is placed in a convenient link in the chain. The slack is taken up by the ratchet and the job is done.