Step 1 - Chop the Redrum door with Jack's axe and then chop the room 237 door with Grady's Finest
Step 2 - Shoot the 42 bottles in the center teleporter area with the Death Machine
Step 3 - Shoot the Grady twins standing in the room 237 door with an upgraded shotgun
Step 4 - Kill 237 zombies near Jack's body at spawn
Step 5 - Shoot the cup at the bar with the Winter's Howl
Step 6 - The typewriter is now accessible and a hidden 237K buyable ending

Thank you!

https://youtu.be/dAjia2kvG-Y