Using a quad equalizer confidently just takes understanding two things: what each band controls, and how to make changes you can trust.
Each band boosts or cuts a range of frequencies. Cutting a problem area usually sounds more natural than boosting everything else to compensate. Make one change at a time, in small steps, and always compare against the original using Bypass before deciding a change is actually an improvement.
The two most common mistakes are boosting several bands at once (making it hard to tell what actually helped) and forgetting to compare against the original after a few minutes of adjusting, by which point your ears have adapted and lost the reference point.
For a faster five-step walkthrough, see the Quad Equalizer Tutorial, or start with What Is a Quad Equalizer? if you want the concept explained first.