Keep it simple
White or wholemeal bread, square/rectangular in shape. Shape is important because it makes coverage with rectangular cheese slices easier.
Toast BOTH sides
Apply strong cheddar, ensuring as much coverage of the toast as possible, else exposed areas start to burn
Place under a very hot grill until the cheese is just starting to brown. Also recommend some tin foil under the grill tray to catch the inevitable meltage.
Apply ketup, but only one slice at a time (assuming you're having 2 slices!), i.e. just before you intend to eat it as the cold ketchup tends to cool the hot cheese and makes it rubbery.
Also good with chopped up chives lightly sprinkled.