1. Idea. It could a general look on how the level plays, some sort of story that makes it work the way it is (for example, something has to be done in point A while you come from point B, but it's in some sort of area that needs to be accessed through C and with something from D) or just a gimmick that I come up and want to make it a recognisable part of the level (think Sonic games). It has to be good enough to make me want to make this level, and trust me, that's a lot.
2. I draw the layout on paper. Usually I draw on checked paper, gluing pages together as the level grows bigger. The looks are minimalistic and only consider the platform construction and most important events (springs, triggers, gimmicks). During this phase, I also think of selection for tileset, music and level name.
3. Using the chosen tileset, I redraw the level in JCS. Due to limitations of some tilesets, I sometimes have to do small redesigns or use various tricks.
4. After the structure's all set, I test the level and see if it's passable. Usually I have to do some fixes because I tend to forget how high or far the characters can reach (this also includes spring jumps). Sometimes I also add more secrets, areas or even whole parts of the level are altered because the old ones are boring or can't be fixed elseway.
5. Eyecandy. I always do it before the final phase because it sometimes changes the masking in significant ways, so I have to do more testing to see if everything I want to be reached really is reachable. Also, I make sure that there are some sort of hints that there might be a secret in a chosen place.
6. The final phase is putting the enemies and goodies in most suiting places. Lots of testing here, not only because I need to see if the level is fair and everything works as planned, but also because it's the most fun part to test. =)