You can directly create a Tag in the left toolbar under “tags”. Right click on “tags” or click on the nut. Then click on “new tag”. In the next window, choose the name of your tag, his color, and click ok. You will find the tags created in the list “tags”. On the example: the perso tag and the RH tag.
To assign a tag to a conversation: Right click on the mail and click on “mark conversation” then “new tag” or select an existing tag (when you create new tags, you will see them appear in the list at that time).