One way to prepare this Italian Sausage (Sweet or Hot) with Marinara is to brown (pan / pot fry) sliced raw sausage in the pot that you will use for preparing the Marinara Sauce. I followed this technique for years, but recently have changed my technique.
Now I prepare my basic Marinara Sauce in large quantities. After the first meal with pasta, I divide the remaining gravy into equal meal size portions (about 4-6 Cups) and then freeze these portions in freezer Ziploc bags. When I am looking to have a meal with meat, seafood or vegetables with my pasta, I defrost the sauce and add the ingredients I desire.
I follow this same process when I am looking to make Italian Sausage with Marinara Sauce. As I am defrosting the frozen Marinara Sauce, I bake the sausages in the oven. After the sausages (about a pound, 5 – 6 links) have been cooked, I slice them to the desire length and then add them to the sauce. I then slow cook / simmer the sauce for an hour or two allowing the flavors of the sauce and sausage to combine. An added benefit to preparing the sauce with gravy in this manner is that the excess pork fat was cooked off during the baking of the sausages eliminating the possibility of a greasy sauce. If needed during the one to two hours that the sauce simmers, add some water for the desired consistency.
After an hour or two, you have a perfectly flavored Italian sauce that serves your needs without too much preparation and leftovers.