Decide the scale for your cardboard barn structure. If you're making a small cardboard barn structure, you'll need less cardboard and tape than if you are making a giant play structure for your classroom. Typically, barns for dioramas don't need need floors because the floor of the diorama is the barn floor. Although a play structure doesn't necessarily need a floor either, you may want to include one as it will provide wall stability for the barn structure.
Use the tape measure and permanent marker to measure the four walls (and floor, if desired) of the barn structure. Measure and draw a door and windows so that you know which side is the front and which are the walls. The roof should consist of two rectangular pieces that are equal in size and two triangular pieces for each end. The roof can be as high as you like and can overhang the sides, if desired.
Use the box cutters to cut out shapes for the walls and roof pieces. If you drew window panes and a door, cut these out as well. You can cut out squares and leave window frames in the shape of a cross within each window. Another option is to cut out a hole for the door handle on the door. Remember not to cut the door completely off, because you'll need it to open at an angle. Cut skinny pieces of cardboard for reinforcing the barn corners if you think they may need reinforcement. If the packing boxes aren't big enough to serve as the barn walls and roof, you may need to cut several smaller pieces from packing boxes to join together.
Tape all pieces together with packing tape. If the packing tape is too wide, use masking tape where the sides and roof join together. Tape the inside and outside of the barn structure joints for extra reinforcement. If the barn structure is part of a diorama, secure it to the diorama by taping the bottom of each wall to the bottom surface. Tape the roof to the top of the barn, unless you want it to be removable to show further detail inside. If the barn is meant to be a play structure, secure the roof to the walls with extra tape and cardboard reinforcement so that the whole structure doesn't collapse as children play inside and out.