Painting a cardboard box may not sound like such a tough thing to do, but it is actually more difficult than you might first imagine. That's because you must paint the cardboard box in just such a way so that the paint shows through but doesn't warp the box and make it lose its sturdiness. This is an issue since cardboard (which is, after all, paper) tends to warp when it gets wet.
Open your can of paint and pour some into your paint pan.
Wet your paint pad in the paint. Don't worry about trying to get lots of paint on it. You want light layers anyway to protect the cardboard.
Spread the paint evenly over your cardboard box until it is completely covered. Do not concentrate large amounts of paint in any one place.
Wait for the paint to fully dry and then apply a second coat. Continue to apply until you achieve the desired look.
It may not be necessary to apply a second coat. It depends on what you want to use the box for and how professional you want to make it look. For the paint, ask the attendant in the store about returns or mismatched paint which may be cheaper than the regular paint. You may want to cover the labels on the box using brown spray paint (let it fully dry first) before applying the colour of paint you want to use.
Do this outdoors, in a well ventilated area. Do not attempt to apply paint indoors without first opening windows, allowing for adequate ventilation.