A tag can be created by retagging any relevant question as such, or for those without enough rep, suggesting edits for retags. For example, in the original revision of your post, you proposed to create the "topological-order" tag, so you can just select any question which is about topological and click the "suggest edits" button on it. You will be lead to a page where you can suggest edits, and you can suggest an edit in this format:
|changes=Add topological-order tag.
And someone with editing priviledges will approve it. However, you can see that this tag already exists.