Here is my achievement guide, for every single achievement in Turnip Boy Commits Tax Evasion.
All of the achievements are ordered and have chapters attached in the video.
For this achievement, you will need to beat several bosses, and get into the bunker.
TO GET INTO THE BUNKER:
- Go northwest of the starting area, to the potato guarding the graveyard. Go in.
- Go north, then west, then north, then east, then north again.
- You'll be in an area with the potato. Dig the grave next to him, revealing a golden shovel, and he'll give you his old rusty shovel.
IN THE BUNKER:
- Go east to the next room.
- Go north to the next room.
- Go east to the next room and obtain a key.
- Go north to the next room.
- Go east to the next room, with the key and hazmat suit, and the diary is here.