Summarize:

Lay the back piece flat on your work surface with a side piece up against it to create a 90 degree angle. Attach the first side piece to the back edge using a screw gun. You will only need three screws to attach the side. They should be placed 10” apart. One at the bottom, one in the middle and one at the top.  Use 1 1/2" deck or galvanized wood screws to prevent rusting. Repeat this for the other side of the house. If a screw gun isn't available, you can use a screw driver. Put the piece measuring 5 ¾” x 5 5/8”, in the interior of the back and the sides. The floor should be flush with the bottom of the back board and side boards. Using the screw gun or screw driver, drill a screw through the side into the edge of the bottom piece and repeat on opposite side. Drill another screw through the back wall, into the side of the bottom piece. Slide the 30" long front piece with slots up against the bottom block, mirroring the back piece. Attach with screw gun or screw driver by drilling through the side, into the edge of the front piece. This is exactly how you attached the back piece, every 10”. Place one screw through the front piece, into the bottom block. Remember, the roof block with sit inside the house when closed to seal the unit shut. The roof block will need to be centered on the underside of the roof. Put the roof piece flat on a table lengthwise vertically. Measure 1” off the back edge and draw a horizontal line. Mark the center of this line which is 4 5/16”. Draw a vertical line from this point to the front edge of the roof piece.  Also mark a center line on the shorter edge (5 ½”) of the roof block, which is 2 ¾”. Place the roof block along the horizontal line. Be sure to line up the center marks on both the roof piece and roof block. This will center the roof block on the roof and allow for equal overhang. Use 1 1/2" galvanized nails and hammer in one on each corner. Nails should be short enough to avoid going completely through the roof. Check to make sure everything looks secure and stable.
Grab the back and two side pieces (these each measure 30" in length). Place the floor. Stand the house upright. Attach the roof block to roof. Nail the roof block to the roof. Place lid on top of standing butterfly house.