I tried it when I went to Prague last summer. They gave us a glass with about 1-2 oz of Absinth, a spoon, some packs of sugar and a book of matches. Thats it. No instructions or anything.
I remember reading a article about Absinth in an old issue of GQ where a sugar cube was put on a perforated spoon and water was poured over it to a) melt the cube and b) cut the drink.
Anyway, after looking like idiots, the waiter came back and showed us what to do. Sugar goes in the spoon. Spoon is dipped into the Absinth and then lit. What you get is a nice little toy
Just make sure you DON'T spill any of the lit sugar into the glass of Absinth (which I did) becaue then you get one glass of fire; it does look cool though
Anyway, after the sugar is melted, drop it into the glass, mix it up and then drink.