I've never tried this personally, but it is something i've been wanting to experience for a while now. I hope my Key Holders will grace me with Their pee sometime soon.
The best way i imagine it is in a bathtub with the slave kneeling or laying down. Peeing in the mouth of a slave is certainly humbling and the exchange of fluids is a beautiful thing. But, the most humbling and degrading to me would be to pee on the slaves useless penis. Show him that his cock is nothing more than a place Her waste goes.