Boost and Manifold Pressure are the same thing - it's just a differently labelled gauge. One islabelled in psi (Boost) and one is labelled in inches of mercury (Manifold Pressure). It's basicly two countries (Britain + Empire and the USA) divided by a common language.
RPM should normally behandelled by the RPM governour, but before constant speed props (= they'll hold the RPM automatically which you're telling them to maintain constant, e.g. 2700RPM) you only had variable pitch propellers, where overspeeding could become a normal issue to look out for. You'd just control the pitch of the propeller with a switch or lever, but there was no governor to maintain a set RPM. The same is true for fixed pitch propellers, even though those were very heavily on the way out, when the war began.
For setting a specific power (e.g. economic cruise, fast cruise, combat, climb) you'd set a certain Manifold Pressure (Boost) and a certain RPM and possibly even a mixture setting (like Auto Rich or Auto Lean) and blower (supercharger) gear.
It sounds all terribly complex - and to a certain degree it is - but after all, it's just setting a value on a gauge and it becomes second nature over time.