Letting a guy cum in your mouth is a beautiful act of selfless pleasure-giving, and is the natural completion of a blowjob. It shows commitment to the blowjob and to the giving of pleasure. It completes the beautiful connection between cock owner and cock sucker and is the ultimate act of selfless brotherhood. Every man who is attracted to cock, or curious about cock, should let a guy cum in his mouth.
The cock fills your mouth and many or most men on the brinking of cumming will tend to want to push it deep. Hands can be used on his shaft to limit the depth if deep-throating is not your thing. For me it's a more intense experience to have the cum deposited in my mouth, rather than direct to my throat.
At the magic moment, he will stiffen harder and will push, then the cum either spurts or oozes and fills your mouth. It feels thick, gooey and warm. The taste fills your mouth and overtakes your senses - it's an acquired taste, not something I'd sprinkle on my breakfast cereal, but in that moment I love it - it tastes of sex and pleasure. There may be a lot of cum, in multiple spurts, which will seem to fill your mouth if you are lucky.
At this moment, you can choose to swallow. Swallowing is natural and makes for the perfect ending for him. It shows commitment to the blowjob and to his pleasure. Gulp down that warm thick cum and you will feel its sticky textrure coating your throat and lingering in your mouth.
As his orgasm subsides and his shaft softens, be gentle with it - this is the moment that many men are extra sensitive in the tip. Withdraw it gently, smile and thank him. You've just completed a joyful act together. When you go on your way and go about your day or evening, the taste and texture will linger to remind you of what you've done.