TL;DR: The Player's Handbook gives you a list of challenges for your players that you didn't realize. I included it at the bottom.
How do you plan a list of encounters? How do you think of problems for the heroes to overcome? Do you just pick a stat block out of the Monster Manual and put that monster in the next room? Pretty easy, but I think most of us are here because we agree that a great DM puts a little more effort in than that.
What if I told you that you were thinking about this all backwards? Like, literally backwards. Maybe you should be starting at the other end.