First of all, you need patience. If you don't, then just stop reading.

You need to complete Coastal Highway. Wait until the credits are over (which will take a while).

When the epilouge starts, start holding RS to the left. Keep holding it.

When it should show the engineer lighting Sgt. Johnson's cigar, it will pan left and show Gunny "playing" with one of the monkeys from the monkey easter egg in Halo 3 Serria 117.