1.I will think about the start of the level first (Usually I will make the start as beautiful as i can, or from the level in The game i played.)
(For example, New Super Mario bros.Wii World 4-1)
2.I will try to make that beautiful picture possible in start of the level.

3.Think of all new mechanisms you can do in LL by triggers. (Wind,Falling ice,or anything does not make sense in real life but if you think the level will be more challenging if you put that mechanism in, count that in.)
4.Think of "the whole picture" of the level. And I gonna put that new mechanisms somewhere in "the whole picture" in the level. The new mechanism can be appear more then once and should make it a little bit different.
5.Make that "whole picture" literal.
6.Decorate.
7.Test for bugs and difficulty of the level.
It's quite hard to tell what I thought because it is too abstract to tell how I design levels while I just have limited vocabulary. But hopefully it can help you.