1. create a few tags. You can do this in the backend either by going to components -> k2 -> tags, click "new" and create a tag; or when you're entering your item, under the category dropdown there's a spot to enter a tag then click "add."
2. Once you have tags to choose from, when entering an article, you can choose from your available tags and add them to the "selected tags" area. They'll now show up under your item.
3. Now activate the tag cloud module. To do this, go to your module manager. Duplicate a "k2 tools" module, then open it. Change your title, choose a position, and under "Basic Options" you'll see a dropdown list next to "Select module functionality." Choose "tag cloud." Down further on the right you'll see a section named "Tag Cloud Settings" where you can make adjustments.
3. The tag will be larger, comparable to the other tag words in the tag cloud, if there are more items with that tag. When people click on the tag word, they'll go to a page listing all those items with that tag. You can choose options for that if you go to the k2 parameters popup. Under "layouts & views" you'll see a section named "Layout & view options for tag listings."