hi, i’m really glad you enjoyed the game! :)
To help you on your way with level design, I will attempt to describe my process without giving too much away ;)
As you will have seen while playing, the first lavel is all about introducing the basic game mechanics in a way that is informative, and (somewhat) mandatory. I always try to show the player exactly what I want them to do, while helping them to do it.
I also try to keep the goal clear for the player, and I try to show them where to go. In A Knight and his Ghost, the player is dropped into each level, indicating that the drop is the goal!
That was the first level, for the second level I try to introduce the advance gameplay mechanics like how death is part of the game, and not a gameover. I try to keep the goal just out of reach and let the player die, highlighting the heart, and showing that the heart is the new temporary goal.
when designing te rest of the levels i always set myself a challenge ie. “How small can i possible make a level”, or “can i make a level in two parts”, or something akin to that, to inspire me and keep me designing something new every time.
When I have a set of levels I always try to rearrange their order, to make the difficulty progression as smooth as possible in my own opionion. A good way to test you levels is to try and break them. Also try to have someone else play your levels, and improve on the points that they struggle with. Dont look for compliments, look for critique.
hope this helps you design fun and creative levels! Good luck and feel free to message me if you need anything ;)