My advice

My advice is a paper note $$

Locations
It can be found inside the Kanawha Nuka-Cola plant, in the first room entered from a roof hatch, behind the large console.

Behind the scenes
The last sentence of the note, "It's in that place where I put that thing that time," is a reference to the 1995 film Hackers. The character Ramόn Sánchez says the same thing while making a phone call from prison.