You begin by gently sauteeing chopped or finely sliced onion in butter and/or or oil. (Adding a pinch of salt and a drop of water softens the onions as they cook so they don't burn.)
Add the rice and let it 'toast' in the soffrito for a few minutes, until the grains turn chalky white. Take care not to brown either the onion or the rice.
Add a splash of wine—almost always white, but red is possible in a few instances. For a very special risotto, you can use champagne. Let the wine evaporate.
In a separate saucepan, bring your broth to just below the simmer. Now add the hot broth, ladle by ladle, allowing each ladleful of hot broth to evaporate before adding the next. Stir from time to time. (There are some cooks who say it's only necessary to stir when adding the broth.) Repeat until the rice is 'al dente', which should take around 20 minutes.
When the rice is cooked, take the risotto off heat. Add a generous amount of grated parmesan cheese and, if you like, a bit of butter. Stir vigorously until the rice reaches as uniform, creamy consistency.
You should normally serve your risotto immediately. But for a slightly firmer risotto, you can cover and let it sit for a minute or two. If the risotto is too stiff for your taste, stir in a bit of hot water or broth.