You'd have to either hook yourself up to an IV drip, or have some type of mechanical device that could consistently push some type of large syringe filled with the methadone. I have no idea whether 1 liter is enough to kill you, although it sure seems like it would be. It's highly unlikely you'd be able to push that much into yourself manually before succumbing to unconsciousness, which may, or may not, be enough to kill you.