Here’s a traditional Swedish farm accessory for gunk-laden soles. The dimensions are not critical, but be sure the edges of the slats are fairly sharp?they’re what makes the boot scraper work. Cut slats to length, then cut triangular openings on the side of a pair of 2x2s. A radial arm saw works well for this, but a table saw or band saw will also make the cut. Trim the 2x2s to length, predrill, and use galvanized screws to attach the slats from underneath. If you prefer a boot cleaner that has brushes, check out this clever project.
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.)