It's often handy to work from a pattern if you're feeling squeamish about designing a headpiece for your costume from scratch. Sewing patterns are a great option if you can get your hands on an appropriate pattern for the costume you're making. Here I've adapted a simple child's hood pattern into a headpiece that is big enough to fit an adult.
1. Trace your pattern pieces onto cardboard and cut them out.
2. Texturize your cardboard by crumpling it up and twisting it, this will give your cardboard a more leather-like texture and make it easier to work with.
3. Cut thin flexible strips of strips of cardboard about 1.5 to 2 in. wide by whatever length you will need (I like boxes that are similar in weight to pizza boxes) and glue these strips one side at a time to the two pieces that you are trying to join. Regular white glue and clamps or clothespins work great for this.
4. Continue adding your cardboard seams as you put all your pattern pieces together. trim as needed.
5. Now you can begin to modify your piece for your individual costume. Here I added more pieces with the same technique by gluing thin strips of cardboard for the seam and then attaching a forehead and jaw piece.
Links to a few handy patterns suitable for costume making:
Vest pattern by the Mother Huddle
Hood pattern by Fabric link
Children's fitted hood/hat pattern by Martha Stewart Living
Baby cowboy boots pattern by Nap Time Crafters (You could adapt this pattern for making any kind of boot-like shoe covers.)