Own the Pyre
This one is simple, just do what a description says:

• Go to the Main Menu
  
• Choose Workshop
  
• Select and buy Pyre (you should have 100 coins)

Own the Guillotine
same as Pyre, but Guillotine (you should have 500 coins)

Own the Iron Maiden
same as Guillotine, but Iron Maiden (you should have 750 coins)

Unlock a ghost skin

• Go to the Main Menu
  
• Choose Workshop and click on the small bottom button with the ghost
  
• Select and buy any skin you like! (now the cheapest one is The Krampus, availavle only for the 150 coins)

Unlock 5 ghost skins
using the method I've written higher buy 5 ghost skins

Hidden achievement
All BOTs survived during the game

While being the Matriarch kill all Sinners without mistake (don't kill any NPC during the game)

Hidden achievement
Escape on the first or second day

Hidden achievement
Escape while being the last Sinner alive

Hidden achievement
The exit code is 666

Hidden achievement
All sinners are alive

All players shouldn't be killed during the game

Spook the Matriarch three times

If the Matriarch discovered that you are not an NPC and sent you to another world, you would become a ghost! While being a ghost, you can spook Mommy to slow her down a bit. It's not easy work, so you need to practice a lot

Hidden achivement
Kill 3 Sinners during the triple sacrifice

To activate the Sudden Sacrifice (or Triple Sacrifice, it's the same), you need to ring the Doom Bell near the Church on the Convent Map and near the other bells on the Monastery map

The Matriarch instantly would have power to send 3 holy soules to hell

note: it's possible to sabotage the Bell while playing the Sinner: come closely to it and hold the F button for around 15 seconds. well done!

Hidden achievement
Find the Golden Shovel

The Golden Shovel makes you dig 2x faster. Unfortunately, it can be found only once during the game and it can be collected only by one sinner during the game (the Matriarch simply can't pick it up)
There are a lot of possible places to find the Golden Shovel (around 8 on each map)

Here are the most popular ones:

The Monastery
the bottom of the library (the right part of the map)


under the doom bell (bell to run a sudden sacrifice), near the froggie



The Convent
near the church and the pillars with numbers


kinda hard to pick, you need to go a long way over the road to the well


it's hinding under the apple trees


note: golden shovel can't be picked up on "the barn" map

Jump scare

There is a frog hidden on every map. You should scare it while being a ghost (the dead sinner)



The frog will turn from this



to that





Here you can see loactions of the frog:

The Monastery
It's located near the big bell in the central part of the map


The Convent
It is located above the pillars with the numbers, near the cemetery


The Barn
It is hiding near the potatoes in the central part of the map