As a follow-up to the French Absinthe ritual I posted the other day, this is the Czech method. This is probably more appropriate, given the origin of the bottle pictured.
Pour the absinthe into a glass, preferably a traditional absinthe glass. Place the sugar cube on a spoon, dip into the liquor, and then rest the spoon across the top of the glass. Flame the sugar and let it caramelize. Extinguish the flames by adding one to two parts water
This method uses half the water of the French method, and so makes a much stronger drink. The caramelized sugar adds a little additional flavor.