Objective: remove video background behind a portrait orientation still image

Here's how I did it:

(I have a picture showing steps but I don't seem to be able to insert it.)

1. Insert main track in timeline
2. Insert portrait still in another track
3. Insert transitions at beginning and end of portrait still
4. Split timeline of main track. Note that the split goes in the middle of the portrait still transitions
5. Insert transitions at beginning and end of main track6.
6. Select the middle portion of the main track (i.e that portion that you want to delete the video from; turns blue)
7. Select MODIFY
8. Select MASK
9. Click on white mask box
10. Click on INVERT – this should remove the video in the background leaving the portrait still
11. Click SAVE under the PIP box

(I used to write freshman biology lab manuals. 'Never overestimate the ability of the students to follow directions.")