well it depends on what style of levels you want to make. try starting of making a auto mario stage. if auto mario keeps failing. than the players might run into similar fails. its all trial and error to make a good stage. which needs to playtested alot which might get boring.

When playing levels, try to find the differences in the physics of the different game styles. For example, in NSMBU, you can triple jump and do a midair spin. This gives you more distance and momentum not seen in any other game style.