My 1st tip is to trial n error in placing platforms, walls, traps etc. The first attempt is to fit level size, platforms wall etc to fit character's movement. If it's too cramped, make some distance, if it's too empty, add some platforms and walls in between.
My second tip is to know your character's limit. You must know how high he/she can jump vertically, how far he/she can jump horizontally, or dash if he/she could perform dash, just to name a few. Knowing this will give good base on level design, and it's essential for designing metroidvanias.
That first tip might be repeated again for other reasons such as balancing, difficulty control etc.