It has fascinated folks for many years.
In America, it was illegal (as are so many great things) until sometime in the mid-2000's.
Anyway, enough of that, here's how you do it....
First you have to obtain a bottle of absinthe. My favorite is Lucid and can be found at many of your friendly neighborhood liquor stores for around $60.
Second, you have to have the correct spoon. It should be slotted and able to balance on the lip of the glass.
Third, you get some sugar cubes.
Now that your tools are ready, pour a shot of the absinthe into the glass.
Balance the spoon on the lip of the glass and balance your sugar cubes atop the spoon.
Many mixes call for one sugar cube, but I put in two. (I like it sweet.)
Begin a slow drip/pour of ice cold water (I mix mine at a ratio of about 3 parts water to 1 part absinthe) over the sugar and into the glass.
You will notice your drink taking on a hazy greenish color. This is normal.
Once the sugar cubes dissolve, allow your drink to breathe for a moment. This will help stir up all the good flavors.
Finish pouring your water in and enjoy!