Place an 8-inch-diameter paper plate on a sheet of shirt cardboard. Trace around the plate with a pen and then cut the circle out of the shirt cardboard with scissors. Using this shirt cardboard circle as a template, cut an 8-inch-diameter circle from a felt sheet.
Place a 10-inch-diameter paper plate on a sheet of felt. Using the paper plate as a template, cut a 10-inch-diameter circle of black felt.
Mix the wallpaper sizing with water in a painter's tray to make a thin mixture. Soak the two felt circles and an uncut felt sheet in the wall sizing.
Roll a sheet of shirt cardboard into a tube with an 8-inch diameter. Secure the edges of the tube with hot melt glue and let the glue set. Use the 8-inch circle of shirt cardboard that you used as your felt template as the top of your hat by gluing it to an end of the cardboard tube.
Squeeze excess wall sizing from each piece of felt. Spread the 8-inch piece of felt on top of the cardboard hat mold. Spread the uncut felt sheet around the circumference of the cardboard tube portion of your hat mold. Allow to dry.
Glue the 10-inch circle of black felt to the 10-inch circle of shirt cardboard.to make the hat's brim. After the glue has dried, attach the felted cardboard brim to the bottom of the cardboard "hat" mold you are making. Pipe clear hot melt glue around the brim to secure. Wearing rubber finger protectors or rubber gloves to protect your hands, spread the glue carefully.
Trim the inside of the hat to fit your head. Cut a small hole with scissors first, then gradually trim the material on the inside of the cardboard tube until the hat fits your head securely.