[WORK FOR ALL CLASSES]
1.) Pick the class called 'demoman' and have the 'bottle' equipped in the melee slot, please note if demoman has already been selected, you must visit your local resupply cabinet to get your bottle.
2.) Make sure to hit it against any surface/enemy player until it breaks.
3.) You now cannot die and can hit people with your broken bottle (titled broken as it breaks the balance of the game)