Go to the Customers page. Mark the checkboxes next to customers you want to tag:
Click on the Tag button and a drop-down menu containing all of your tags will open:
To tag your customers simply click on any tag from the list. If you need to create a new tag, scroll down to the end of the tag list and click on Add Tag:
A popup will open where you have to give your tag a name, choose its color and once done, click on Save:
That's it - tag has been created and you should see it in your list of tags from now on:
How to create, edit and delete a tag from the Inventory Settings
Go to Settings - Inventory, and scroll to the bottom until you see Tags.
Here you can see all of your tags, Edit them, Add New or Delete them entirely.