Follow the main quest, once you get to the Yawning Portal Tavern (place you can speak to Durnam the barkeep).
Just jump into the well behind you (should probably accept Durnam's quest first)
Jump into the well, don't click the sparkles to take you down.
Once it loads up the in game achievement Trust Fall will pop, then so will the achievement.
This shouldn't take you more than 10 minutes to get.
If you do click on the sparkles, just turn around click the bucket to go back up, and jump down again
If you need to track whether this has counted in game or not ( in case achievement isn't popping for you, could be lagging, or move to a quieter instance and try again)
Journal > Achievements > Exploration > Trust Fall.
No check, no achievement/doing it wrong.