There is plenty of info on this already on the forum, I am guessing you have not tried using the search feature yet?
You just need to add {ORDER TAGS} in your [ORDERS] section, then create an [ORDER TAGS] section that contains tags relevant to order tags. You can view the available tags in the help on the right side when editing template.
Here is something to get you started, it describes also how to add order tags to the receipt: