Instructables
I, personally, have been annoyed when I don't have a sleeve for my NES cartridge's.  So I have made a replica for the original sleeve that usually comes with any new game like Super Mario Bros.
 
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Step 1: Materials/Tools

Here are the materials you will need to make this:
Glue(I used Elmer's.  It dries very fast.)
Cardboard(Make sure it is very thin.)
Pencil/pen
Tape(optional,if you don't want to hold it forever.)

Tools
Scissors
Box Cutter(Helps in the future,trust me)

Step 2: Measurement

Picture of Measurement
Here are the measurements for each side of project

Front and Back Measurements:
Total Length: around 5 3/8 inch
Width: around 4 7/8 inch depending if you messed up or not
Thickness: 3/4 inch

Then layout all measurements on cardboard.

Step 3: Cut Pieces to Size

Use box cutter first to trim the line then use the scissors to cut the cardboard.  If you use the box cutter to cut everything, it will be tougher and you could cut surfaces or even yourself.

Step 4: Glue/Tape Pieces Together