1. Arrive in Sanctuary for the first time.
2. Once you are inside Sanctuary, head past Scooter's and continue past Dr. Zed's Clinic until you reach a dead-end.
3. If you look to your right, you should see Claptrap slightly behind a wall (I believe there is a Yellow Exclamation Point giving away his position as well.)
4. Talk to him and complete the "Claptrap's Secret Stash" quest.
5. Immediately following the quest, while still aiming at him, there should be options for "Talk" and "Give Me Five!"
6. Press the key/button for "Give Me Five!" which will cause Claptrap to raise his right hand (to the player's left.) Melee his hand to unlock the achievement.
PLEASE NOTE: It is possible to miss this opportunity, as you can only choose "Give Me Five!" ONCE. So, if you do not melee his hand in a certain amount of time, he will roll away, and you will have to either restart the game (without saving) and repeat the process, wait until you get another opportunity during a later quest (such as Wildlife Preservation,) or do it during a different playthrough.
ALSO: This could possibly be a glitch, but when I chose "Give Me Five!" while playing as Zer0, he immediately rolled away without giving me the chance to melee his hand. Maybe it's because Zer0 uses a sword? ~~Shadow Hado
11:09, 2 October 2012 KiKzR