I think the problem is with time, or tense, in the sentence. It seems the idea is to consider all effects of the plan so the task can be finished, which implies the work is in the future. But, the word "use" is in past tense, implying the either the plan was used before, or the plan was previously selected to be implemented. If the plan was used before, or previously selected, than why would it be imperative to consider all the effects now? Maybe it is better if you leave the word use out, then you have "To finish this task it is imperative that all effects are considered in the step-by-step plan". From this sentence, consider what additional information is communicated by including the word "use". The word developing may be better than used, as in "To finish this task it is imperative that all effects are considered while developing the step-by-step plan", or do you mean ""To finish this task it is imperative that all effects are evaluated in the step-by-step plan selected". Good luck, English is the only foreign language I'm studying.