I started with a few quick sketches of early level ideas, working closely with the programmer to ensure it was achievable.
This also helped from a design point as I could see if gaps and hazards were avoidable by the player. (E.g. the characters jump distance was far enough to clear gaps).
Following the “kishōtenketsu” philosophy, I suggested we built upon the jump mechanic by programming it so the jump distance was relative to how long the jump button was pressed. This allowed me to design more elaborate jumping puzzles.