Download and print door and window templates; cut out. Position door and window templates on one box as desired, using a ruler for placement. Trace templates onto box with a pencil. Slide a self-healing mat inside the box, and position it behind the windows as you cut out the panes. Position the self-healing mat behind the door; cut the curved top edge, the flat bottom edge, and down the center. Fold open along straight sides. Repeat tracing and cutting on second box, using window template only.

Assemble bottom of the first box (the one with the door): Fold in two opposite flaps, generously coat them with hot glue, then fold the remaining two flaps over them. Hold flaps in place until the glue is set. On the open end of the box, cut the flap off of each long side. Cut off half of the flap from one short side, as pictured; fold it in. Secure flaps to side of box with a line of hot glue. Hold flaps in place until glue is set.

Begin making the roof: On the top end of the second box, use a ruler and pencil to draw a line from the top center of one short-side flap to one bottom corner; draw a second line from the top center to the other bottom corner. Repeat on the opposite short-side flap. Cut off one long-side flap, as pictured; set aside. Cut along a penciled line to remove a triangle shape from one short-side flap, as pictured; score the second line. Repeat on second short-side flap, cutting along one line and scoring the other.

Cut long-side flaps from the bottom of second box. Cut one short-side flap so that when the box is assembled and stacked on the bottom box, the holes will line up. Secure the flaps to the side of the box with a line of hot glue. Hold flaps in place until glue is set. Coat the top of the bottom box with a generous layer of hot glue. Position the bottom of the top box over it, and hold in place until the glue is set. Use one of the reserved short side flaps and coat one side of it with a generous layer of hot glue. Place it, glue-coated-side down, as pictured, on the "floor" of the top box for reinforcement.

Cut the roof piece (21 1/4-by-46 inches) from third box with a crease down its center when folded short-end to short-end. To attach roof, fold along the scored lines on the short-side flaps at the top of the house. Glue long flap to the folded triangles from the short sides. Generously coat the folded-in pieces with hot glue, and position the roof over them. Hold in place until glue is set. (Don't glue other side of the roof closed. Your cat will enjoy peeking out from under it.)