When the water returns to the boil, throw in the pasta. Stir occasionally.
For dry pasta, start tasting after about 6 minutes (fresh pasta after 2 minutes) - fish out a piece of pasta and bite it - it should be resistent (al dente or "to the tooth"), not crunchy, not completely soft.
Drain and serve. Some cooks reserve a bit of the cooking water to use if the sauce is too dry.