Here's what I found on the internet (and this is how I remember it as well, so these instructions should be correct)
Original Trilogy - there is only one "easter egg" for the entire trilogy and its on the trilogy set with the 4th bonus disc:
The “Star Wars Trilogy” DVD set contains an Easter egg on the Bonus Materials disc. On the Main Menu, highlight “VIDEO GAMES AND STILL GALLERIES” and press Enter on your DVD remote. When the next menu appears, on your remote punch in the number 11, followed by the number 3, followed by the number 8. Exactly how you punch in the numbers may vary slightly depending on your equipment, and punching in the two-digit number 11 may require that you first press something like a greater than or equal to 10 or 10+ button. If you do everything as required by your particular equipment, a gag reel will start up after you punch in the number 8 on your remote. The gag reel runs for about 4 minutes 45 seconds, and in between outtakes and the like, you will see DVD credits.
For each individual Prequel movie (Episodes 1, 2, 3) the instructions are the same:
Insert Disc 1, navigate to the OPTIONS menu, then use the arrows to highlight the THX icon. Then punch in the number 11, followed by the number 3, followed by the number 8. Exactly how you punch in the numbers may vary slightly depending on your equipment, and punching in the two-digit number 11 will probably require that you first press something like a 10+ button or a greater-than-or-equal-to-10 button on your remote. (On my oldest DVD player, I have to press the Enter button after each of the three numbers, but my two newer players don't require this.)