There are a lot of sites for making mockups, but I like this particular one because
a) you can start without registration
b) It’s easier than most of analogues
c) you can set click listener to any element (when you click element it leads to another screen). So you can demonstrate the logic of program.
As my voice sounds just terrible on recordings, I just used subtitles where necessary.
Also it is hard to see anything on a small YouTube screen, so I recommend you to make it 480p and Full screen.
And video is one minute longer than the required maximum, but result demonstration is the same in the beginning and in the end of the video, so just skip it in the end.