How can I correct things just by guessing what you’re doing, while you also guess what another guy’s doing? 
Is that the first tutorial series you’ve checked out to learn Unreal? If so, there are many well known YouTube channels that you can watch beginner tutorials from. There are also some beginner tutorials you can find on this website as well 
Okay I found the tutorial you were talking about. It’s not a making from scratch kind of tutorial, the guy just explains how the code which he provided works. You’d need to know some basics before trying to make it yourself without any guidance.
Here’s a nice starting point:
Keep in mind that even after you gain experience, you’ll still struggle to edit other peoples’s codes since you won’t have an extensive grasp of the context unlike the person who created it.
Hope I was able to provide some guidance on your confusion 