Pushing the cork into the bottle is the best way to open a bottle of wine without a corkscrew. You can also use a lighter, a shoe, or even a bike pump.
Keep the bottle of Champagne at a 45-degree angle and turn the bottle, not the cork. There should be a fizzle, but no pop.
I peel off the little plastic seal affixed to the top of the bottle, toss it out, place the cap back on, and squeeze away.