First, use a square to draw a rectangle on a cereal (or similar) box. Cut it out with pinking shears. Draw a rectangle in the center of the cardboard shape — I used the width of a ruler as a guide — and cut it out with a craft knife.
Cut paper into quarter-inch wide strips. Weave them in, one by one, trimming the edges with scissors after each one is in place.
OR: Cut all of your strips before you start weaving, and then weave them in. Either way works!