How do you guys know how to make stages good? It's too hard for me to make anything cool or fun.
There's several ways to go about it. You could remake some of your favorite stages to see what it was about them that you loved. You could experiment with the maker until you find a quirk or technique you hadn't thought of and expand on it. You can also play other stages and experiment with ideas those stages had.
As I mentioned before, Mark Brown is the perfect suggestion.
He did a standard Mario vid:
https://www.youtube.com/watch?v=dBmIkEvEBtAHe also did a Mario Maker vid:
https://www.youtube.com/watch?v=e0c5Le1vGp4Both are quite helpful in designing a good Mario stage.
My main suggestion is to find one thing you want to do in the stage. Start by showing, in a safe environment, the basis for the entire stage. Then slowly expand on it. Take away safety nets, force it multiple times in a row without breaks. Find an advance version of that technique. Play with it.
Oh, and get a play tester. Don't tell them anything going in. You'll get interesting feedback that way.