METHOD
On some bottles it’s a good idea to remove or cut the foil on the neck of the bottle. To remove the twisted-wire hood or not is a personal choice, it’s easier with the hood removed.
A tip is to swing the blade along one of the two lines where the two bottle halves are joined together. It is said that the glass is weaker along these lines. 

Hold the bottle in a 45 degree angel in one hand and the sword in the other hand with the back of the blade facing forward. Put the sword against the bottle.
The back of the blade is most commonly used since this is the way since the tradition was born. However it doesn’t matter which side of the blade is used.

Swing the blade firmly along the bottle, when it hits the top of the bottle the glass will brake and the cork along with the top of the bottle will shoot out.

Since there’s pressure in the bottle the risk of getting glass in the wine is almost non existing.

Beheading champagne is not a matter of strength, one can see on the cut on the bottle if the swing of the blade was too hard or not. If the right amount of strength has been used the cut is straight.

Some suggestions before choosing wine
Sparkling wines that are not produced according to the traditional champagne method often has too weak glass and could break when struck by the sword. Your always best off with genuine champagne, but cava will also do fine.

Safety
It's important to always be carefull while sabering.

It's a good idea to use a linen napkin to hold the bottle and protect the hand from shards in case the bottle should break. If the glass would break, remember to be relaxed in your hand and do not try to grip the shards.

Always remove the foil from the cork, otherwise shards might spray in the direction of the sabreur.

Make sure that your audience is behind you, the cork will fly away and might hit someone.