When you create a post (or edit an existing one), the first field is Title. The second field is Free Content Tags. Just type the tags you want into that field.

As you type, you'll notice a revolving circle at the end of the field. That indicates that the system is trying to lookup/find the tag as you type it. Sometimes, that lookup takes a few seconds, so be patient. If the field provides any listings as a dropdown below the field, you can click on one or more of them to select them. If the system doesn't provide any matches, then you're probably typing in a new content tag. Go right ahead.

After you save the post, you'll be able to find it not only by its specific URL, but also listed under all posts for that tag. For instance, you can find all posts with the tag "School Portal" at http://npstac.org/free-content-tags/school-portal.