WordPress.org

1. Upload the theme-preview folder and its contents to your wp-contents/plugins directory.
2. Activate in the Plugins menu.
3. Visit your site with a special parameter added to the URL

Add query variables preview_theme and/or preview_css to your query string. For example, if you have a theme named "My Theme", which is installed in your wp-content/themes/my-theme directory, add the theme's directory name to your URL like this:

http://example.com/index.php?preview_theme=my-theme

Sometimes, you create a new look for your site by just making new CSS, but you keep the existing PHP files intact. In that case, you con use the preview_css variable instead of preview_theme, or use them both together, like this:

http://example.com/index.php?preview_theme=default&preview_css=my-theme