Here's my seven-step approach, plus a bonus tip
1. Ask your buddy to hold your dick for you while you take a piss. Simple as it sounds, holding another man's dick while he pisses is surprisingly enjoyable for both parties. This is a good way to introduce pissing as a fun and sexy experience.
2. Once your buddy is into watching you piss and feeling your piss flow through your cock, invite him to lick your piss slit clean after you finish. That one drop of piss won't be flavorful enough to turn him off. On the contrary, the subtle flavor of that one drop will help your buddy to develop a taste for piss. My man found it quite fun to lick my piss slit clean, and he actually made it a habit even before he realized that I had harder-core piss sex in mind.
3. While he's licking your piss slit clean, surprise your buddy with that final squirt of piss that tends to linger in your waterworks. If he's shocked, you can always feign innocence, because every man knows the phenomenon of a surprise leak after pissing. Whether your buddy is shocked or delighted, definitely express how exciting it was for you to feed him that squirt of piss.
4. Once your buddy is accustomed to taking your final squirt of piss, stop your urination a little bit early so that you'll have two or three final squirts of piss for him. Again, let him know how turned on all this makes you. You'll likely have a raging hardon, so he won't be surprised to know that piss sex makes you horny.
5. One day, I went into the bathroom to take a piss, and my man followed a minute later to drink my final squirts. However, it was one of those urinations in which the flow was reluctant to get going. In other words, my man came in for a squirt and my bladder was still full. I began pissing in his mouth and just kept on. He couldn't take my dick out of his mouth without getting drenched in piss, so he kept swallowing. As a courtesy to him, I repeatedly broke my flow so that he wouldn't get overwhelmed or start choking. Breaking the flow can be uncomfortable, but it's important when a man is still learning to sip an entire piss load.
6. Once you've established long sessions of piss sipping, encourage your buddy to jerk off while he drinks. The more he eroticizes the experience, the hornier he'll be for more.
7. Ultimately, your buddy will guzzle your piss as quickly as you spray it down his throat. In other words, he won't need you to break your flow. The first time my man drank me down in a single go, I was so turned on that I immediately began fucking his face. There's really nothing like pumping your cum down a piss-slicked throat.
BONUS TIP: If your buddy is already into spit sex, save up some spit while you're pissing in his mouth and then feed him the spit as a reward, like a doggy treat.
Like any fetish, piss sex seems exotic until one actually tries it. Granted, piss is an acquired taste, but so is wine, beer, and cum. Luckily, in addition to being fun, piss is good for you (do a Google search on the health benefits of piss drinking). And it's a great way to broaden your mansex horizons.