Lay the brown kraft paper down on your kitchen table.
Trace a square on the paper corresponding to the size you wish the bottom (floor) of your bag to be.
Trace a second square around the first, corresponding to the height you wish the sides of your bag to be. The height of the bag will be the distance between the two squares.
Cut around the outside square on all four sides, giving you a perfect square of kraft paper. From there, cut smaller, equal-sized squares from each corner (cutting from the outside of the larger square to the inside of the smaller at each corner) until you have formed the remaining paper into a "+" shape (like the Red Cross symbol).
Fold the flaps of the "+" inward and tape them together, forming a bag.