The correct answers are: Please chop the onions while the water is boiling. We need to sanitize the counters before the health inspector arrives. When the timer rings, take the soufflé out of the oven immediately.

In time clauses referring to the future (introduced by when, before, after, or while), we use a present tense, not a future tense. Therefore, "after the guests will finish" and "while the cake will bake" are grammatically incorrect!