Let's do this step by step

At the bottom of your post, where you see your name and the votes, you will see "Edit"

Click on Edit (sometimes it works the first time, sometimes you must try a few times. When it works, the box with the original text will appear. At the bottom of it, you will see the thin box with "expect more"

Add
writing fiction fantasy

Click on Port or Update

You should now have 5 tags:
expect more writing fiction fantasy

Let me know you did it and I'll give you the next step
:)