Up High, Down Low Achievement
Up High, Down Low15
Gave Claptrap a high five
You'll first get a chance to high five Claptrap in Sanctuary where he's located in the north west corner of the map. He'll also offer you the mission "Claptrap's Secret Stash".
You'll first need to complete the mission he hands over to you, which isn't very hard. Once done, turn it in and then follow him. He'll only walk around the corner to another NPC. Once he does, get close and you can then press to give him a high five.
Simply press and Claptrap will then put his hand up in the air. Now aim up and press to melee his hand for it to count as a high five. It may take a couple of goes to connect, but once you do, he'll put his hand back down and the achievement will unlock.
If you miss it this time, there's plenty of opportunities later on where you can ask for as many high fives as you like.