Buried 50 bodies.
One that will come to you fairly easily over time, burying 50 bodies is easy but requires a little work and a lot of time to pass.
Every day, the Donkey will bring a corpse to your morgue front steps at dusk. There’s a few things to know about this.
First, the Donkey will never bring you more bodies than your morgue can hold (see the upper right hand corner of the screen when you’re inside; you start at 1 body as capacity, and can add more by crafting more pallets and preparation tables. Exhumed bodies do not count towards this limit.
Second, at some point the Donkey will go on a little strike and require a few things from you before he’ll start bringing bodies again. This is a bit of a hassle, but from then on, he will only bring bodies if you have paid him in carrots (paid in advance by putting them in a box nearby his delivery spot). However you can use this to your advantage when you want to do other work and not worry about popping back in to deal with fresh corpses - simply leave the box empty when it runs out and he’ll wait at the box with the newest body until you put in more carrots.
Thirdly, burning bodies at your ‘crematorium’ (funeral pyre spots)
also counts as burying a body (as evidenced by your reward of the Burial certificate for payment). does NOT count as burying a body for the purposes of earning this achievement; however, it is still a preferable way to get rid of a body that is unfit for putting in your graveyard, since burning it will produce a burial certificate which you can turn in for money.
Tossing a body in a river does NOT count, and doesn't give you a certificate either. Only toss bodies in the river if you can't burn bodies yet (or no longer care about money from certificates).
So, basically, just keep taking care of the bodies the Donkey brings you, and eventually you’ll unlock this achievement without much thought. The graveyard can hold somewhere around 88 bodies, so assuming you don't even exhume any bodies, you'll earn this long before you fill your graveyard.