Left 4 Dead 2

Throughout the campaign you will come across big green boxes that contain helpful items. Simply open five of them and the achievement is yours.

EDIT: Just an extra on where I have found them so far:

Riverbank:
1 - After you enter the shop and head upstairs one can be found in the second room on the right.

2 - After the upstairs bar one can be found in the store room on the right before you go down the stairs.

3 - In the upstairs on the motel, there are some more stairs that lead up to some guns. Just off to left at the top there is a door into a small room where the box can be found.

4 - One or the other of the two pavilions either side of the dance floor at the wedding.

Underground:
1 - Up in the top room, facing out to the Jazz Club, after to cross a plank of wood between buildings.

2 - Near the start of the underground section, surrounded by orange fencing.

Port:
I have yet to find any here. I suspect that there isn't one, if anyone can confirm then that would be great.

As Kirst Fenix says they aren't always in the same place everytime, and may take more than one playthrough to get a total of five open.

If anyone knows of anymore locations the boxes appear tell me and I can add them in.

This is extremely easy and I will assume you are going for this achievement only. Start up a Single Player game with the following settings:

Campaign: The Passing
Chapter: 1: Riverbank
Difficulty: Easy (Or anything else)
Character: Random (Or anyone you prefer)

After watching the intro video follow the course of the map until you get inside the souvenir shop (the first building you can enter). When inside, head upstairs and search all the rooms, there will always be a footlocker in one of them. Note that the footlocker might be after the corridor in the bar area.

Once you have found the locker, open it, grab one of the items inside and press Start > Call a vote > Restart Campaign.

Repeat until the achievement pops up.

All credit goes to RoosterTeeth.
Like the video says the foot lockers spawn random so they might not be there the first play through. It says it will prob take 2-3 playthrough to get the achievement. Also you dont have to get all 5 in 1 playthough.
Hope this video helps

There are large containers which look like trunks scattered in different areas through out the campaign. Simply go up to it and press the X button to open them. This is easier to do if several friends are helping you look for them, as some can go unnoticed and are not usually found in the same location every time.
Note: this does NOT count if somebody else opens the container in the game, you have to do it yourself.
If you don't know how many you have left to open, press the start button, go down to achievements and it should tell you.