Measure the painting to determine the size of the box you need. It is best to wrap the painting first in plastic wrap or bubble wrap before measuring. The wrap will provide extra padding and a moisture barrier.
Add 1 inch in each direction---length, width and height---to your measurements to determine the box measurement. This will provide you a 1/2-inch buffer on all sides of the painting.
Mark a rectangle the size of your height and width measurements for the box on the cardboard.
Measure outward from each corner of the rectangle the distance of the height of the box in both directions, then make a mark. Connect the marks on each side. You will have a rectangle in the middle with four flaps, which will be the sides of the box.
Cut out the rectangle and attached flaps with the utility knife. Keep it in one piece.
Run the knife over the lines of the rectangle to score them.
Fold up the four flaps and tape the corners with packing tape.
Repeat the process to make a top for the box. Add 1/4 inch to the width, length and height so it will fit over the box bottom.