To add a tag anywhere within an Interact server, just click on the Add Tag icon at the top of the page. You will then be shown a page to enter the following details:
Heading - here enter a brief heading for the tag. This will display as a link to the full tag on your 'Tags' page.
Note - add a note here that describes the reason for the tag. Also here you could paste in some relevant content from the page that is being tagged.
Add tag for - this is an optional setting. By default a tag will only be visible to you, but using this setting you can also make it available to others.
Category - you can also optionally set up categories under which to store your tags. If you are adding a lot of tags this will help you to better organise them.
Also on the 'Add tag' page you will see an a link to 'Tag external site'. You can use this to add a tag to a site outside of your current Interact server.