You carve out a hole slightly larger than the sausage. You hold the roll at approximately a 45 degree angle, while spinning it. Thus the hole remains in place, while the bread moves around it. As you are spinning it, you put the toppings in. Then you shove the sausage in (after acquiring enthusiastic consent).

Sauce-like toppings (mustard/ketchup/etc) are essentially put in one end before the sausage, and as the sausage is pushed in, it spreads the sauces throughout the whole sausage.

Some others (fried onions, etc) get put in with a long spoon before the sausage.