Hi... It is "step by step"... but depends what you want to say...

Hello Olive, 'step for step' and 'step with step' are not correct. 'step in step' is an expression used to refer to exercises for benches. The correct expressions used are rather : *Step by step: which means gradually, with care and deliberation. * in step with: which means in agreement with someone or something or at the same level or speed as someone or something. * step in: means to become involved.

Where did you get those phrases from? 'Step with step' doesn't really make any sense, neither does 'step in step'. Step for step can be used in a sentence such as: "I followed him step for step," meaning - I walked exactly the same as someone else I was following. Though I don't think that's even a very good example. It's more common to use those words as 'Step by step", such as: 'These are step by step instructions on how to put the table together.' Meaning, instructions which show you every single 'step' involved in making a table. "There is a step by step guide available." "Follow it step by step" - meaning, actually follow those steps individually, dont skip any stages, etc.