Multistep form is a jQuery plugin for multipurpose step form wizard.
Copy-paste the stylesheet <link> into your <head>
Place the following <script> near the end of your pages, right before the closing </body> tag, to enable them. jQuery must come first, then Multistep.js.
After downloading, extract the files into the directory where your website is located. Your directory will look something like this.
You'll notice that there are two sets of the files. The min means that the file is "compressed" to reduce load times. These minified files are usually used in production while it is better to use the unminified files during development.
Instead of having a css folder, you will find that the download instead contains many .scss files which contain the styles of individual components. Unfortunately, the browser cannot interpret Sass, so you must have your Sass compiler compile the scss/multistep.scss into a regular CSS file. At this point you can link this newly outputted file in your HTML page.