Place two sheets of 60-by-40-inch plywood next to one another to create a 60-by-80-inch rectangle with the 60 inches as the height and 80 inches for the length. Ensure the seams are flush. Draw the carriage shape on the plywood. Use as much of the plywood as possible on both the height and width.
Cut out the carriage shape using a jigsaw. For tighter cuts, use a rotary tool. Sand the cut edges using medium-grit sandpaper. Place the cutout on top of two more pieces of 60-by-40-inch plywood. Trace the shape and cutout the second carriage using the jigsaw and rotary tool. Sand the edges with sandpaper.
Place the first set of cutouts on your work surface and place a metal bracket over the seam where the two pieces meet. Position the bracket 3 inches down from the top edge of the carriage. Secure it with two 3/4-inch wood screws on either side of the seam. Place a second bracket 3 inches above the bottom edge of the carriage, over the seam. Secure it with more wood screws. Place a third bracket in between the first two, secured with additional wood screws.
Place the second set of cutouts on your work surface. Place the edges flush and place a metal bracket 3 inches in from the top and bottom edges of the carriage, securing them with wood screws, as you did the first set. Draw a rectangle in the space between the two brackets for a bed entrance. Make the rectangle 1 inch above and below the two brackets. Make the width 36 inches wide. Sand the cut edges. Cut the rectangle out with the jigsaw. Sand the cut edges to prevent splinters.
Paint the side of the two carriage cutouts without the brackets. Use flat wall paint or acrylic paint. Add details such as wheels, windows, and detail work. Paint two pieces of 18-by-42-inch plywood using the same colors as the carriage pieces. The 18 inches is the height and the 42 inches is the width. Let the paint dry overnight.
Prop one carriage cutout up with the painted side against a counter or sawhorse. Position one of the 18-inch pieces so the 3/4-inch side of the smaller plywood is flush with the backside of the carriage. Square up the edges so it sets against the outside edge of the carriage. Place an L-bracket on the inside corner of the two pieces, positioned 2 inches down from the top edge of the smaller piece. Secure the bracket with more wood screws. Place a second bracket 2 inches up from the bottom edge of the smaller piece of plywood.
Place the second piece of 18-inch plywood on the other side of the carriage. Secure it to the back of the carriage using two more brackets spaced the same as the first two, securing them with wood screws.
Prop the second carriage cutout up against the two 18-inch plywood pieces with the painted side facing out. Secure the smaller pieces of plywood to the carriage using four more L-brackets positioned and attached the same as the first four.
Place a 42-inch dowel rod in between the two carriage cutouts. Center it along the width of the carriages, right above the top metal brackets. Hammer a 2-inch nail through the outside of the carriage through the dowel rod on either side to secure it. The dowel rod provides extra support to the structure.
Set up a twin box spring and mattress where you want the finished bed. Place them directly on the floor. Lift the completed carriage up and set it down around the bed.