Spread a fine layer of mustard on both parts of the bread, so it's not too dry but not too moist. The mustard will "glue" the meat and the cheese to the bread and they won't slide off the sandwich.
sauce goes on the buns, onions pickles, that kinda shit on the bottom below the meat, cheese directly on meat, tomatoes on that, leafy shit above that to stop the bun getting all soggy.