If your sauce is watery, the easiest time to fix it is before cooked pasta, tender beans, or delicate greens have gone in. Those ingredients can overcook while you wait for the liquid to evaporate.
Simmer uncovered in a pan with enough surface area, stirring as necessary to prevent sticking. Watch the consistency rather than relying on a fixed time. A wide pan reduces differently from a narrow saucepan.
Taste as the sauce thickens because salt and other flavors become more concentrated. If it is already too salty, reducing it further is the wrong direction; you need a suitable unsalted addition or a different plan.
When the sauce coats a spoon in the way you want, add the cooked components and heat through. Keep a little water nearby in case you have gone too far. Restoring a small amount of moisture is easier than rescuing burnt sauce.