Fry onion in buter, add about 75g of risotto rice per adult (aborio or carnarlio rice) and stir around in the butter, add stock a bit at a time and stir for about 20 minutes until the rice has absorbed enough stock to get the consistency you want.
Other things: add them earlier or later depending on how long they need to cook.
As per Trillian's instructions. But I'd cook the sausage under the grill first and also lightly roast the tomatoes and then just add them a few minutes before the end. If the sausages aren't that spicy you could add a little dried chilli and perhaps some fennel (goes very well with sausage) at the end (or roast the tomatoes with a little chilli and fennel).
Also, not sure how you feel about cooking with wine for DC but I'd add some wine at the begnning before the stock and also add some garlic to the onion (leeks and celery add a nice creaminess to risotto too).