Two things to remember and you'll do fine.
One, the new rank will PROBABLY look identical to the old, except that it will have another stripe on it. Put the new one on exactly the same way as the old one. If in doubt, look at what the person putting the other one on is doing, and don't be afraid to smile and say "Like this?"
Two, he will have been wearing the old rank up to that time; you may even be removing the old one. There will be pin-holes left where it was. Just use the same holes and it will be in the right place.
On an unrelated note, don't worry at all about embarrassing him or yourself. Not only is HE being honored with a promotion, YOU are being honored as a participant. No one is going to give you a hard time, and as long as you take it even half-serious, you CANNOT get it wrong. Think of this... the last two times I remember Presidents throwing out the first pitch at a baseball game, they actually threw a pitch. However, I remember once long ago, Queen Elizabeth threw it out by dropping the ball into the pitcher's glove from where she sat. Being English, she wasn't expected to be able to throw a baseball. The point of which is, you're the Guest of Honor. Nothing you can do will be viewed as "wrong."