Near the end of your animation, you seem to expect something that won’t happen: guides won’t cut a face, they just provide things you can snap other geometry to. You still have to draw the rectangle to start making the opening.
Also, as you struggle with in the animation, you should turn X-ray off immediately after getting the walls vertical. X-ray enables snapping to things that normally would be hidden, and that is why you had to work so hard. In his example, Dave turned on X-ray so that he could snap to corners inside the building while straightening the wall.
Finally, in your model you have a lot of copies of things spread out over a large distance from the origin. That can potentially aggravate loss of precision. It’s better to make components or groups and use tags/layers to control which are visible than to scatter contents all over the place.