Insertion of the thumb into the anus with the other hand raised above the head in an over-arching style such as an ape might perform when walking upright. The trousers and underwear should be at least at half-mast to allow the insertion of the opposing digit, or preferably you should be completely naked, to make it a classic Mogilla Gorilla. Use of the "oo-oo-oo" sound adds greatly to the effect.
The girls at the party were really prudish, so I dropped the Magilla Gorilla right through the middle of the lounge room. That livened things up!