Normally, a fob is attached to the other end of the watch chain. The fob could be any number of things just like a key chain decoration.

The fob is then put in one of the waistcoat pockets and the watch in the other; the chain hanging across the front of the waistcoat. (Couple of different ways to do that.)

I have a pocket knife for mine, or I have a mounted coin that is probably for a key chain.

If the watch is to go through the buttonhole of the lapel, with the watch in the breast pocket, you might think about using a more decorative and small sized fob.