Working on one side at a time, glue and nail the side to the back. Apply glue and drive three 1-5/8-in. nails into each shelf, attach the other side and nail those shelves into place to secure them. Clamps are helpful to hold the unit together while you’re driving nails. Center the top piece, leaving a 2-in. overhang on both sides, and glue and nail it into place. Paint or stain the unit and then drill pilot holes into the top face of each side of the unit and screw in the hooks to hold your ironing board. Mount the shelf on drywall using screw-in wall anchors.
You can also use Google image search to research a project. Unlike the PDF search, the image search provides a photo of what the project will look like, which is helpful in determining if you want to pursue building it. By doing an image search for how to build a step stool, you will have a ton of options that all lead to woodworking plans of various quality. (The how to part of these is important—otherwise you’ll just get pictures of step stools.)