Practice the cake cutting. Cut, cut, stab (as I like to call it).
Many years ago we had a couple that was cutting the bride's cake and the groom absent-mindedly fed himself cake before feeding it to his new bride. It was pretty funny then (and now)! If you want to see a clip of it, here it is. Please keep in mind that it is just raw footage so it won't be the masterpiece you are expecting from us!!! And really do yourself a favor and skip to :45 into the clip.
We understand that you don't cut this sort of cake every day and it is kind of weird to have to cut it with your hand here and his hand there and wait for the photographer to take the picture. BUT IT IS JUST CAKE; JUST CUT IT HOW YOU WANT!!! Guys, please do keep in mind that your bride's mouth might not be as big as yours and don't feed her a bite that you would take, but maybe half of the size.
No one ever knows how to cut the cake without instruction, which is fine. Just wait until your instructors have moved away until you start cutting the cake. That way your pictures and video won't have half of the serving staff in them.
Here is an instructional article that we found that may provide some assistance before the wedding.