We cannot give you the details for the synthesis of Benzoyl Peroxide because that compound is a very sensitive and powerful high explosive. It may sound a bit strange, but a very powerful explosive is a great acne treatment! (I can only imagine how that was discovered. A pimply faced chemist was working with the stuff when it detonated on his/her face. A few days later he/she noticed their acne was gone! lol. If only that was the real story).
So while we cannot tell you how to make benzoyl peroxide, I will say that when the body absorbes the peroxide, it converts it completely to benzoic acid. The human body then metabolizes the benzoic acid into hippuric acid. So if you were to measure a benzoyl peroxide exposure, you'd have to run the urine sample through a GC/MS and look for hippuric acid and not benzoate.