You need to enter the graveyard mini-biome.
This is usually a surface biome, requiring five or more gravestones close together, though the achievement may work underground as the biome can exist (without spawning mobs however)
Gravestones are obtained when a player character dies.
Just mine them up with a pickaxe, and place five together, and you have a graveyard biome for the achievement.
I recommend removing them as soon as it pops, as most things obtainable in graveyards aren't good past the early game, and aren't worth the hassle if they are actually an upgrade, as the enemies could be difficult for a new player at this stage.