You need:
3 or more players of legal drinking age in your area,
some palatable alcohol, and
a "blue bin" of 1000 Lego-compatible building blocks, or whatever approximate number you can gather. The selection of blocks should be reasonably diverse in size, color and shape.
Choose a player to call the first round. The game has 26 rounds, and the first round uses the letter A.
Each round, the caller for the round chooses a noun of less than four syllables, preferably something concrete rather than abstract - unless the caller likes to get punched in the arm by increasingly inebriated friends.
All players, including the caller, must then draw from the available pool of blocks and build something that is recognizable as the called noun. There is no hard and fast standard for recognizability; this kind of thing should be worked out by rough consensus amongst the players.
The last player who successfully builds the call must take a drink. That player becomes the caller for the next round, which uses the next consecutive letter of the alphabet that the new caller can remember.
This game has an elegant structure that tends to distribute the damage fairly well - the caller has a bit of a time advantage, considering how long it takes to think of something good. However, too much complexity will kill any drinking game, especially one that requires handling small objects, so Lego Marathon leaves plenty of room for the kind of spirited conversation that's comprised of the soul of great drinking establishments.