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,

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).