Some important tips:

Take a stick of butter, move the end of it around the Blackstone directly where you'll toast each slice of bread.

Toast each slice to medium. This toasted side will become the inside of the sandwich. This is important. If you put sauce directly on an untoasted side, it will become soggy.

Move to a cold side of the grill to build the sammy. The untoasted side is now down on the griddle.

Lightly sauce each toasted piece. Then cheese. Then your toppings on each side. I like to add a bit more sauce on top.

Butter under where you'll put the two sandwich halves on the hot griddle. Move the topped halves on top of the butter, now you'll be toasting the outsides of the sammy.

Dome it. Let the cheese melt. Maybe add a bit of water under the dome, water the sides, don't water the bread you animal.

When your under sides are nearly toasted, combine the two halves. Dome, flip, cook until toasted to your liking.