[FS.Kezz]

It should be very similar to when you learnt your first race, except:

+ You know the units and their roles
+ You know how the game works in general
+ You don't know things like hotkeys, correct compositions, and the macro technique (i.e. inject/larvae and chronoboost)

I would personally use TeamLiquid (or your preferred build acquisition method) to find a macro build that 'generally' works in all matchups from the races you're unsure of. Get fluid with executing them, then branch off into other playstyles.