Step one: Cut the box so it is about 2.5 inches in height and cut off the flaps on top. This is so the cat can easily climb in and out.

Step two: Put the bag or litter box liner in the box like you would when placing a trash bag in a trashcan. This is so litter does not escape the through any holes in the box and the wetness doesn't destroy the box after a few uses.

Step three: Tape the bag to the bottom of the box making sure there are no holes. This step is optional but recommended to hold the bag/liner in place.

Step four: place the box where you want it to go and pour the litter in your box. (reverse these if it is going inside a cage)

Note: make sure your cat knows where it is placed. Cats extrinsically know how to use a litter box but can't use it if they don't know where it is!