Position the screw of a large C clamp so that the head of the screw is pressing against the inside of the brake piston.

It is possible to use your hands rather than a C clamp, but it is much more difficult.

Slowly tighten the C clamp, forcing the brake piston back into the brake cylinder until it is completely depressed.

Make sure that you work slowly, or else the piston will force brake fluid out of the master reservoir and all over the motorcycle.