You are flying around on a nice VFR day and the engine suddenly starts to run roughly - what would you do?
You land at the first airport from which your girlfriend can get a cheap airline back home
Seriously, it is system dependent. Pull out carb heat, mixture full rich (unless already at high altitude), pull alternate air (if fuel injected). Full pump ON. The rest depends on available engine instrumentation: EGTs, CHTs, fuel flow rate. There is even a failure mode on some Conti fuel servos which gives you a rich cut and the only way out is to lean drastically.
You have to know the plane and all its systems before you get airborne.