Hi there. I don't know the video instructions you refer to, but usually the process is:
1) Subscribe to the add-on and wait for the download / file check
2) Select an empty save slot or start a new game
3) Check via "manage add-ons" that only the desired/compatible workshop items are active
4) At the top right of the start chapter selection screen, switch to "add-ons"
5) Select the map corresponding to your active add-on, usually sorted by subscription time

Hope that helps and have fun.