1. Open layer details for the layer you wish to add a text field to.
   
2. Click on the Edit layer icon.
   
3. The Layer editor dialog opens.
   
4. Go to the Fields tab.
   
5. Click on the Add new field located at the bottom of the existing fields list.
   
6. The content editor will open, make sure you set the Type attribute to Long text.
   
7. Set other fields as applicable.
8. Save by clicking OK.

Now your layer contains a new text field.