Question: how to use hydrogen peroxide to clean grout?
Answer: Pour some hydrogen peroxide into a spray bottle and spray the grout to get rid of mold and mildew.
Reapply if necessary, scrub and rinse.
Use a small stiff-bristled cleaning brush and apply the homemade grout cleaner to the the area and scrub the grout. ... 
Let the solution sit on the grout then rinse and wipe dry.