So, First the best way to learn for editing Levels is to make first a race game; You`fe just to made the layers, springs and some ammo.
Tip Using a tileset with different up`s and down`s;
The second level will be a Gem hunting one. You have to insert Gemz (Best is 150 to 200) and learn to make way`s which will stering you to the start of each selfes..(Very good for Hard Single player levels! See in MEZ`s Queen of board..)
The third level you made can be a CTF (Capture the Flag) or a battle level; Insert ammos hidden way`s (Using Layer 3) and (in CTF) the flag base..
Then you can make Single player levels;
Insert enemies endbosses food ammo and all things you can find in the Event list. The best single player level is the one which isn`t to hard and to easy; there must be places where you must think to find the right way and there must be used the Event`s for using Easy, Medium and Hard Mode. Besides Multiplayer (Internet, Cooparation mode…) If you do this I`m telling you here you`re (I Think) on your way to be a good Level Editor…
Eat your lima beans, Johnny.