I'd say you should start soft. Try just a drop or getting your back pissed in a bathtub for example. You'll start feeling more comfortable and then you can move on to stuff like getting pissed inside or in your mouth. Also, it helps if at least for the first few times, the person who's going to piss on you has had a lot of water to drink. Piss will still be piss of course, but it wont be so strong smelling and yellowish, which in my case, makes it nicer.