Put mayonnaise on your sandwich (I'm using a panini grill so I don't but anything on the other side. But if you use a pan put the mayo on one side and butter on the other)
Put your choice of cheese! (I like cheddar personally.)
I like to put a small amount of garlic. It melts with the cheese and makes the sandwich taste kind of like garlic cheese bread. Up to you though.
Add some onions for crunch!
I like to add some 1/2 shredded turkey or ham. It's yummy. :) Once you're done putting all the stuff on your sandwich, add a little more cheese to make sure everything sticks together.
Then grill it up! :) once it's done enjoy your amazing, grown up grilled cheese sandwich. Also, experiment. Add different things onto the sandwich. :)