The first step is to create the tag in the setting. See the article on "creating a tag". Once it is created can start using it on contacts. There are a few ways to add they tag to a contact.
Click on the contact you want to add the tag to.
On the tag field on the contact, start to type the tag you want and a list of tags will pop up. Choose the one you want.
3. Once you have chosen the tag you want, click "UPDATE CONTACT" red button.
ADDING OR REMOVING TAGS TO A LIST OF CONTACTS
From list of contacts screen you can add and remove tags to multiple contacts.
On the left side of the screen, click on the box for the contacts you want to add. You can click the "First" box to select all.
3. Once you have selected the contacts you want to tag or remove the tag, click on the red icon in the right hand corner to assign the tag.
4. Select the tag from the drop down and click "Assign" or "Remove"
This same process is available on the Outreach stats for the people that have received you text or your email.