This Seeking Tech tutorial will explain how you can get the Mostly Dead trophy and achievement.
This will unlock when you “use the Fake Death pill.”
You can get this as soon as you find your backpack at the beginning of the game. Make sure you have the Fake Death pill equipped to your Items menu and then use it to unlock the trophy/achievement.
Keep in mind you can use the Revival Pill to resurrect yourself shortly after using the Fake Death Pill if you don’t want to restart from your previous save.
Check out the video guide below if you need additional help: