Hey Danilo!
That part of the video did go by kinda fast but I think I have a pretty good explanation for making the guides.
The vertical is perfectly fine where it is. It's function is to set the max height (the head in this case), and the minimum height (the feet), as well as measure the body (head lengths).
The horizontal line sets the width of the figure. It should probably extend from the tips of the fingers on the left, to the elbow on the right. (I think) Marc set his at the bottom of the pelvis but I don't know if that was intentional. It can help you lay out the proportions similarly to the proportion chart assignment.
After setting those two lines, you should be able to create a bounding box that encompasses the whole figure. This will help you figure out the proportions better as you will know that the figure shouldn't extend outside the box.
If you're finding this assignment difficult, I would suggest re-watching the video a couple times, without doing the assignment at the same time. I find it helpful to really give the video my full attention if I'm having trouble understanding, and then do the assignment while watching the video the second or third time. It might also help to the cylinders/line of action/skeleton first if you're still having trouble.
I hope this helps. Good luck!