So my timetable would be:
Season 1:
Wage war
Muster units A in provinces were I am able.
Season 2:
Wage war
Improve units A to 2xp
Muster units B in provinces where I am able.
Season 3:
Wage war
Improve units: A to 3xp, B to 2xp
Muster units C
Season 4:
Wage War
Improve units: A to 4xp, B to 3xp, C to 2xp
Muster units D
Continue mustering until you have enough units, then continue improving units until they have the desired level..... It will be expensive though, since improving units are on active service.
Season X
Move units to same province
Create field army
Move units off to war
Perhaps the best course of action would just be to train them to skilled and march them to battle to die or gain further experience?