I find it is very easy. Most guys will allow their **** sucked because they want it sucked. And most do not really care who does it. They may have preferences, but push comes to shove let the willing person suck away.
Put ur hand down there over his pants and there it goes by itself
you dont, go to a glory hole and there are many ***** for you to suck without saying a word to anyone. Sit in the booth and wait for a **** to poke thru and you can start sucking. Good luck.