I find designing puzzles really hard too, and I still haven't worked out if there's a 'proper' way to do it. I just add and move elements around, playtesting constantly to see if what I've made is impossible or not. For instance if I make something that's too easy, I remove a block and see if it's still solvable, that sort of thing (repeat over and over again). I figure designers like Draknek and increpare must have a better approach, or their various incredibly thoughtful puzzle games would probably take decades to create.