Here is how you can add a new pick list field to a form that already exists:

1. Open the form you'd like to add a pick list to.
   
2. Click the edit (pencil) icon in the top right corner of the form.
   
3. Click the "FIELD SETTINGS" tab.
   
   
4. Hover over the new field you've added and click the pencil icon.
   
5. Click the "Setup pick list" button.
   
   
6. Add your list of options and click OK.
   
7. Click 'SAVE'.
   
   
8. Click DONE in the top left corner of the Edit Form Settings.