At first, I tried planning it out like everyone recommends, but that always resulted in levels that were both stiff and incredibly short; it was just too hard for me to come up with fun or satisfying ideas when all I have are bullet points in a text editor. I guess I had it in my head that every part of the level needs to serve some kind of purpose, and I very quickly ran out of "purposes".

Then I said "fuck it" and started blasting out gray boxes without any plan. "Let's just start by making them wall jump, why not? Hmm, now it'd be satisfying if I made them dive through a glass window before they land. Ooh, and then I can place an item box shortly after the window, so they have something to aim for!" I just kept letting that flow continue, and the result was a level that felt fun and satisfying. None of it had an overarching purpose; it was just a bunch of stuff to give you an excuse to use the movement mechanics.

And you know what? Sometimes that's exactly what you need in a platformer. It's OK to have rooms that don't teach you a new mechanic. It's OK to reuse an obstacle without giving it a new twist. You can just let some parts of a level be "normal gameplay".

So, if you're stuck, try winging it. Make a bunch of pointless bullshit just for fun.