Do this before you have your meal and it's always best if the bottom has evacuated the bowels first. Empty and clean is good!
Bottom needs to be face down and relaxed. He (top guy) will need LOTS of lube, sensual foreplay helps a lot too, and go slowly. Once he gets his head inside, he should stop while you adjust to his thickness. Slowly he will then start to fuck your tight end and the pleasure for the bottom will be realized once he has opened you up a bit.